Men and women cannot be friends. /Harry/

The worst that can happen is pain and suffering resulting from lost friendships, watching them go through divorce, a carry affect into your own relationship leading to divorce, a breakdown at work leading to unemployment, leading to a terrible ulcer that results in intestinal damage, that wipes away your savings and forces you to sell your house to pay to keep living, which forces you to homelessness, hunger, and drugs, including a dirty need, which gives you the final disease.

It depends on how tight you are with the guy. If I was pretty tight with the guy and the cheating evidence was 100 percent credible, I'd for sure mention it to him. Now, this should not be a "to catcha predator/cheater" intervention. Just man to man say what you've heard and ask if it's true.
People cheat for a variety of reasons, for men in prolonged situations it is usually because they are either insecure or there is something not right in their relationship.

What is your motivation for talking to him? Is it your friendship with his spouse? Sometimes cheaters are just caught up in the excitement of something new...the new, added attention and take their eyes off the long term goal/security of a marriage, and need someone to add some clear perspective.
Or, they are really in an unhappy situation and should have never been with that person in the first place and might need to make the tough choice to scrap it. But I'd recommend couple's therapy before a divorce for sure.
Either way, if you do bring it up, keep it mellow. Listen more than talk.

He convinced me, Stay out of it!

How good of a friend are you to each individual. I mean... if you had to pick one of them, who would you be closer to?

If the guy: Tell him you know and he needs to cut that shit out. Tell him to man up and either divorce his wife and let her find her own way... or he needs to end it and confess his misdeeds to his wife. She can make her own decisions from there.

If the girl: Tell her what is going on. You are no friend to her if you don't warn her that her hubby might bring home some unwanted disease from his screwing around. Every day this continues... her life is at risk.

*edit

I would be royally pissed and probably cause physical harm to "my friend" if they knew my OL was running around on me and did not tell me.

I'm closer to the guy.

But my fiance is closer to the girl.

Your move.

My move is exactly what I told you.

You are buddies with the guy? Then talk to him.

If he refuses to admit he is ****ing up then it falls on you to let the wife know.

Might it ruin a friendship? Yeah. But really... a guy who ****s around on his wife is no one *I* would want to be friends with anyhow.

How would you feel if his OL wound up with syphilis or even HIV because of this?

Shit... the guilt would eat me alive.

Let your fiance do whatever she wants, then. You keep quiet.

You're not "really" friends with the other girl, if the dynamics of this "couple relationship" are like 99% of the ones I've ever known. The four of you get along and so you kick it. You're friends with the guy and your chick is friends with the other chick. The crossover connection is tenuous at best.

The end result: start looking for new friends.

Its obvious you want to say something. Just get it over with man. I would just stay out of it myself. I guess it would all depend on who i am closer too.

I would ask myself, who would be my friend still if that couple broke up. If you dont think the girl is going to be around after they are done with, its not worth it. You lose 2 friends for something you didn't do.


Like i said, just get it over with, you obvious want to tell her. Only 2 choices, do or dont do. Pick one and go. lol

Tell him to quit cheating on your other female friend, tell him he needs to tell her or you will have to..

Same advice I gave earlier.

It seems like you're seeking absolution from us. Well, seriously, who the hell are we? Do what you're going to do.

You obviously want to tell her, go do it. While it's the wrong thing to do, and she won't believe you if she doesn't want to, knock yourself out.

At the end of the day, no matter how cliche in sounds, you have to be true to yourself. If you would want to be told if it was you, then tell her or talk with him. If not, let it go. If you lose a friendship over doing what is right, then the true depth of your "friendship" with this couple is questionable (on their side).

Completely different subject than what my advice pertained to. I'm sure 99.9% would say 'yes, I would want them to tell me'. However, I wouldn't expect my friends to come right out and tell me. I would expect them to make sure I was aware of what was happening without jumping right in the middle of me and my wifes relationship/problems/ etc. There's a big difference between the two.
The problem I see here is you have a scumbag for a friend. You would never be in this situation if you took responsibility for who you choose to be friends with. I'm sure there is some way to make sure she is aware of what he is doing with out letting either one know you had anything to do with it. Whatever you decide to do dump the scumbag friend you have. Even it it means you can't hang with the wife for awhile. If all of their friends start distancing themselves from him that in itself may get her pointed in the right direction about her husbands character.

I usually don't butt in like this but...

If the guy doesn't know that you know he is cheating, then the first step you take is to have a talk with him. Go about it however you want; casual, heart to heart, whatever. You have to get a feel for what is going on with him. How he reacts to your "talk" will tell you a lot.

You cannot/must not skip this step...

Guaranteed that after you talk with him, his reaction will tell you everything you need to know and consequently will help you make your decision...