I would tell. I do not subscribe to the philosophy that cheaters should be protected. I also think it’s a load to say that you would be “involving yourself.” She already involved you.
I have a feeling if the genders were reversed some of the answers would be different.
Homie the Clown?
Don’t say anything now, it’s great material for the best man speech.
Generally, IME and from what I’ve read/heard, the cheatee will promptly and loudly dump the truthful friend and defend the louse, no matter what genders are involved. Of course, two years from now, the cheatee might return to the dumped friend and say “you were right.” Are you a gambler?
If you don’t tell, then in two years, the cheatee, in the throes of divorce, will say “you knew and you didn’t tell me?”
So lose your friend now, or lose your friend later. Your choice.
But either way, do keep us informed as events occur!
You are the smartest man on the internet.
(Too obvious?)
It’s like you guys want to be featured in my sig…
Speak now or forever hold your peace. I say speak. Everyone has a right to make improtant decisions with all the pertinent information in hand.
“I don’t wish to interfere, and I’m not sure what your relationship status was at the time (you did say they broke up a lot) but I want you to know that X, Y, and Z happened. I won’t repeat this to anyone else, but I felt I should tell you before you commit in case it’s improtant to you.”
Then never mention it again.
A friend of mine got engaged while in the Navy, & his fiancee was living with his mom for a while while he was at sea, & apparently hit on another friend of ours. I’m not sure how much got back to friend in the Navy. But that relationship went back & forth for years. They didn’t get married, then they later flirted with getting back together, then he moved across the country…
The friend she hit on seems to have fallen out with Navy guy–maybe for other reasons.
I think you may as well tell him. A friendship ‘protected’ this way can fail eventually for other reasons. And he may forgive you for telling him sooner than for not telling him.
Well, sure, if you assume that the friendship is going to be destroyed anyway, may as well get some comic value from the mess. But I think it’s better to tell him more privately–but away from weapons.
Do you have a mutual friend who’s a gossip that you can tell ‘in strictest confidence’?
1987: Party at a friend’s place – I need some smokes, so I say I’m walking up to the store. Friend’s girlfriend comes with me, and halfway there tackles me to the ground and starts making out with me. I go along for a few seconds, then tell her “No, you’re off-limits”.
Friend must’ve known she wasn’t the most faithful one in the world, because when we get back he asks me if anything happened. I lied.
1992: Out at a bar, I run into girlfriend again. She makes another move, and I asked if she was still with friend. She was, so I said “no” again. She moved on to some other guy and spent the evening with her tongue down his throat.
Maybe five years ago: After not seeing him since the early 90’s, I run into friend. After reminiscing for a bit, I come clean about that girl he had dated all those years before and how much of a tramp she was…how she was constantly making moves on me…how I saw her cheating on him with other guys…
Needless to say, trampy teenage girlfriend was now his wife. Oops.
Dammit! I knew I was being too obvious!
Hal, you do have a certain talent for keeping things…interesting. 
Yes, but what was his reaction to your story?
“Yep, I knew she did all that, but she’s worth it both then & now.”
OR “Damn I wish youda said something then. I haven’t been able to afford the divorce I’ve wanted since a month after the wedding.”
OR “I KEEELL you!!11!”
ETA: my reaction to the OP: Tell your friend now. All the facts as simply as you can. And accept what happens. Darn few secrets stay secret for long. And even fewer when at least one of the parties is a flake or a drama queen (of either gender).
My advice is not start threads when you’re drunk.
It went basically like this:
“…blah blah blah tramp slut ho-bag blah blah blah…”
“She’s my wife now”.
“Oh…huh. So, whaddaya think the Giants are going to do this season?”
That was the end of it, it never came up again, and I haven’t seen him since.