That doesn’t make sense. What if the wife is there? Is it ok then? What about other friends? If I show up and just the two of them are there, I should go home? 
That was for SoC, not SX.
Do whatever you want. If it were me, I might tell the friend that I would cover for him and then tell the wife the truth if or when she called. After all, trust obviously isn’t that big of a thing to him and what’s good for the goose is good for the gander.
A little story: Many years ago I was dating a guy - let’s call him Dave. Now, I had two good girlfriends that I hung around with quite a bit, let’s call them Jill and Jane. One night Jane comes to me, upset because she was at a party where Jill’s boyfriend had sex with another girl (Jill was not at the party.) Jane wondered if she should say anything. I immediately said she should, as soon as possible, that no friend should ever withhold information like that. So we called Jill, and she came over, we told her the whole thing. She freaked, and broke up with her boyfriend.
The next night, Jill and Jane sit down with me, and tell me that my boyfriend Dave had been cheating on me with a girl they knew for about six months. They had known the entire time but didn’t know if they should tell me, and they felt so guilty after my strong reaction the day before, they felt it was time to come clean.
Well after that evening we were all single for a while, but the moral of the story is that I’m still very good friends with Jane (Jill moved away eventually), whereas Dave came off looking like a complete asshole to everyone we knew. Ten years later, everyone still knows what he did to me and what an asshole he is. Don’t be the asshole. YOU HAVE TO TELL THE TRUTH.
That’s all fine and good, but there are a number of factors in this story that are crucially different from yours.
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a child, (won’t someone think of the children?)

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marriage
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the families
I’m not trying to be a jerk, but I honestly don’t see why this is confusing to you. If I knew that someone was having an affair and I happened to find my self in the their (the couple having the affair) presence, I would take myself out of their presence.
So, yes, even if other friends are there and you are not alone with them, but you are still socializing with them (even in a group setting) you should go home. You should go home especially if the wife is there and the two of them are there. You should go home if just the two of them are there. Unless you condone them having an affair, which is up to you.
All of that going home business assumes that you don’t want to move to another part of the bar or that it would be really bizarre if you did that. In fact, you don’t have to go home at all, just go somewhere else.
Criky, why do people think children are so ignorant? Why the hell would it be OK to cheat and lie about it just because you have a child? When I was young, I used to pray for my parents to get divorced. They were not fooling me. It took them waaaaay too long to come to the same conclusion I did.
As to marriage, is it OK to cheat and lie about it to stay married? HELL NO! The guy’s wife should be able to make her own decision about whether she wants to stay with a man who cheats on her.
The families? If seventeen is “of age” then it is the girl’s responsibility to choose her partners wisely, and if her parents don’t approve, then tough. Kick her out of the house, whatever.
The point is YES HE IS A BASTARD for helping his friend. I don’t understand why it’s OK to LIE and cheat on your wife (and possibly bring home disease or get the girl pregnant or have someone else find out and tell his wife/the girl’s parents/the entire town but hush hush for the sake of the children) but it’s not OK to to tell the TRUTH because someone might get their feelings hurt. HUH?
I don’t mean to heap coals of fire on your head but your “friend” has already make it pretty much impossible to be around both of them. If the friendships haven’t already died, they’re poisoned and just haven’t collapsed and started stinking yet. That sounds harsh and you didn’t choose it but there 'tis. The cheaters are the ones who chose cheap thrills over everybody else. In other words, you need to face that the friendships may be over no matter what you do. You’re best choice is figuring out where you stand in the wreckage.
Your friend can’t be believed about just making out. Sorry, but no way he’s jumping through all these hoops just for some soulful kisses and furtive grouping. He has zero credibility for integrity, y’know?
As someone who was in the wife’s shoes (minus child), my worst sense of betrayal came from the folks who let me remain in ignorance of the health risks I took by remaining with my cheating husband. Feeling humilated and foolish was bad enough. For some reason the possible exposure to HIV, herpes, etc. seemed enough to trump their–I dunno–tact? embarrassment?
Even if you don’t tell the wife, Bruce, flatly refuse to cover for the husband. This whole mess is gonna blow sky-high sooner of later. Just avoid his company, even in groups. Refuse to play along or lend your face to his behavior. What he’s doing is wrong, wrong, WRONG so don’t let your integrity–or reputation for it–get sucked down with him.
Good luck, Bruce. What a painful mess.
Veb
An anonymous letter tells you only that either someone wants to screw with your head/life and make you miserable, or that someone doesn’t have the guts to tell you the truth and be questioned about it. It merely sows unhappiness and mistrust without giving any real answers. If you tell someone non-anonymously, you risk losing them as a friend or whatever, but at least they know who it is that’s making these accusations and can decide if they trust you, compared to some anonymous letter.
I’ll chime in with the other posters who advised not to tell the wife. That’s between him and his wife.
Get yourself out of this situation as soon as possible. Tell your friend you absolutely do not want to be involved in any way, shape or form with his affair. Tell him you don’t want to hear about it, won’t lie for him, or give him any advice about it.
If he’s truly a friend, he’ll respect your wishes. It’s just too bad he doesn’t have the same respect for his wife.
If you weren’t friends with the wife, it’d be between the two of them. But you’re friends wiwth the wife, and you’re letting your friend be hurt.
The husband isn’t your friend. He’s a manipulative creep. Near as I can tell, the wife is your friend. You owe it to your friend not to let her be hurt like that.
And it’s not just between the husband and wife. If the husband were a decent human being, that’d make sense – but he’s a manipulative creep, not a good guy.
Consider saying to her, “Look, I don’t want to lose you as a friend, but there’s some real fucked-up stuff happening in your relationship right now, and I’m not real sure how to deal with it. Let me know if you want to talk about it. If you don’t want to talk about it, that’s fine, but I hope you can understand why I’m awkward around the two of you.”
Don’t worry about giving the husband a chance to tell her first. You’re not denying him shit: he’s had his chance to tell her, or not to have anything to tell her in the first place, and he blew it.
Be a good friend.
Daniel
Well, duh. Why would you want to be friends with a loser who cheats on his wife with an underaged girl?
As far as being friends with the wife…you are not being one right now. She is being hurt, you know it, and you don’t want to tell her because you might feel bad.
Suck it up and do the right thing. You should lose him as a friend, and hopefully, the wife will be grateful enough to you to appreciate you telling her the truth.
She is living a lie right now. You owe it to her to tell her what’s going on. What happens next is her choice, but at least you will have done what you needed to do as a friend.
I honestly can’t even conceive of the logic of this attitude. The fact is, it’s NOT between him and his wife, because his wife is totally ignorant of the fact of his cheating*. Christ! She is not making any choices here. Her husband, and people who know the situation but don’t tell her, are making decisions for her. It’s complete and utter bullshit.
The wife deserves to know the truth about her husband. She is married, but the marriage she believes she has is not the one she’s really in. Do you really think ignorance is bliss? What about the health risks? What about the fact that she’s saddled with a complete shit for a partner, and doesn’t know it? IF she knows and decides to stand by him, that is one thing. For you to take that decision away from her entirely is unconscionable.
Tell him you’re going to tell her if he doesn’t by a VERY NEAR DATE. Then tell her, if he doesn’t. It’s awkward and painful for you, but a damn sight more awkward and painful for the wife to quite possibly find out later, and know her husband was sheltered by a bunch of cowards.
If you’re not really friends with his wife, you don’t have an obligation to tell her jack, but refuse to lie for him anymore. And since you are his friend, I’d urge you to try to have a talk with him about this. Maybe he’s just thinking with his dick–I wonder if he’s considered what would happen to his family if his wife found out.
Inferring from what you’ve written in the OP, the wife already knows or suspects that her husband is fooling around.
Now, all she needs is more proof.
…and YOU’RE it!
Tell your “friend” that you’re done playing along. He should come clean to her or, if she asks you, you’ll tell her what you know.
and then Bail.
Immediately.
The person who should have thought of all that is the person SCREWING AROUND ON HIS WIFE. His actions directly force the wife into being the ONLY ONE who thinks of these three things…which makes her look like an ASS to her ex and his new girlyfriend because they think that all is fair in love and war…and none of the three things directly affect them.
When a man finds something new to thump, the last thing that gets blood is his brain. IMHO.
Been here, done that…was the only one to think of the kids, family, marriage. Now I am the “woman from hades” according to the ex and his new wifey. So be it. At least I took care of my priorities before I took care of my hormones.
Tell the wife. Don’t let her sit there and get used, abused, and some possible diseases in the long run. The pain will be worse if she finds out it went on behind her back. Don’t worry about the man…because he isn’t worrying about anyone else but himself anyway.
I agree with so much that’s already been posted - - that I’ll just answer your original question with…yes, yes you are.
Out of curiosity, who thinks that is “OK to cheat and lie about it just because you have a child?”
It seems you’re implying that someone here said this. I haven’t noticed it being said. Could you point it out specifically?
I concur.
The question at hand was about BD’s conduct, no? IIRC, he didn’t do any of the cheating. He did lie, which was clearly wrong. Whether or not BD should inform asshole’s wife is the question.
Fair enough, however, this does not necessarily establish an obligation on BD that outweighs his obligation to protect and promote the family of asshole.
As I metnioned before, keeping mum is predicated on the premise that the affair’s ended posthaste.
Marriage is about more than just two individuals. It joins together their families as well. I was also speaking of these two families, (the wife’s parents, syblings et al and asshole’s parents syblings et al).
I don’t think that you have to understand something that’s not so.
It’s more a question of what course of action will bring the lesser harm to the family.
Cheating Husband is already bringing harm to the family. It is now up to Bruce to give the wife the information she needs to deal with this harm.
Very true.
I should take this time to iterate again that the direction to BD not to inform asshole’s wife is predicated on the premise that the affair is ended posthaste.
If it is a continuing affair that will not be immediately ended then I think that BD should inform asshole’s wife.
Well, BD has an obligation as a friend to act in what he sees as asshole’s best long term interests.
BD has this same obligation to asshole’s wife as well, and by extension the daughter.