Category 1, would feel obligated to alert the innocent spouse that the cheater is taking some nasty risks with his health. I would certainly not allow the cheater to make me a party with her whining to cover up for her public indiscretions.
Category 2, would want the man to know what his wife is up to.
catagory 2 would keep my mouth shut
Category 2, and I wish he’d kept his mouth shut about it. I wasn’t going to divorce him over a one-time (I assume) event – newly married, baby on the way. I didn’t need to know, and I never quite trusted him again. He was a bit of a drama king (there are such things) and I think he got off (pun?) on the “confession” and “forgiveness”. Hell, he cried; I didn’t. Son of a bitch. He’s dead now.
Yeah, and if you had gone down a little farther you would have seen this post as well:
Wow, you ARE a hard-ass! 
Even so:
[emphasis added]
Forgive me if you didn’t mean it that way, but to the casual reader it sure sounds like you are saying that, given your experience, you would not wait for any proof if the anonymous note fit with your existing impressions.
Taken as a whole, the quote - doesn’t sound a lot different in practice from “would dump spouse because of anonymous email”.
My point is that an anonymous email has little credibility, because there are lots of reasons for sending one quite aside from it being true - for example, spite. I myself would not dump an SO on that basis without “additional proof”.
In contrast, contact from an identified source has more legitimacy, if it comes from someone - like the OP - with no obvious reason to be spiteful.
No; if I already have my suspicions, I would take it as a sign that I’m on to something and look for additional proof elsewhere.
I’ve found that once you’re “in the know,” things you pretty much missed or ignored in the past tend to jump out at you, and finding additional proof doesn’t require much effort.
And yeah, once my suspicions are confirmed, I’d have no compunction about showing her the door. My experience is, people don’t change. If they weren’t worthy of your trust in the past, they won’t be worthy of it in the future.
Update
She called and texted again and again. I ignored and replied that I was busy. Finally figured that I might as well face the music. So I did call her back eventually. Boy was I confused as she seemed i) inebriated and ii) rambling on about C being a good and she being lucky to have him and she knows that it is not always the case in marriages and something about her sister’s husband being an asshole. Finally, she asked if anyother of C’s colleagues were there (yes, though AFAIK I am the only one who saw the kissf). Strangely, nothing about what I intended to do nor pleas as to what that should be.
I am just going to sit back and hold tight.
What a mess. She’s a mess, she’s making a mess, and she’s making sure that as many people as possible get splattered with her mess. Did you tell her to stop calling you and trying to drag you even further into her mess?
Yep. Stay as far away from the mess as possible.
AK84
Any update on what’s been happening, or if you’ve been in contact with Chuck or Wilma at all?
I had lunch with Chuck a couple a weeks ago. Nothing from what I could make out and we had a lot of work related stuff to cover anyway.