I handled this same sort of situation by telling my best friend if he went through with it I would tell his wife, because I thought a lot of her. It worked out in my situation and he didn’t cheat (AFAIK) but your mileage may vary!
:smack: You can ONLY make matters worse by speaking a word of this to anybody. It will ONLY make things worse. There is nothing to lose by just staying out of it, and there’s nothing to gain by being involved with it. The question shouldn’t be “is it worth it to him to fuck up his marriage by sleeping around?”, it should be “is it worth it to YOU (and his snitch band-“mates”) to fuck up his marriage by talking about it?” There’s nothing to be gained. Whatever is going on with this guy and his marriage and his side girl (IF that’s even what she is), it will eventually be resolved one way or the other without the intervention of anyone else.
One cannot be “a good friend” and behave like that.
Find better friends. Ones that don’t saddle you with dilemmas about their personal life.