I would never confront the "other woman," though she is as guilty of cheating as he is. If I were to confront anyone, it would be the married cheater. Perhaps it could be something as 'innocent' as, "I ran into Mary Jones yesterday, who said hello. How do you know her?" And should he give anything less than the truth, reply with "I see" very seriously and with a pained smile. However, the relationship expectations of other people is not usually any of my business, unless I am related to them. So I would probably say nothing - but slide them a dirty look even once in a while. :o) Perhaps even a "tch tch" shaking of the head.

Since confronting the OW, unless she is married to someone you know, is pointless, confronting the MM to give him a chance to tell his wife before you do (as opposed to stop before you tell) would be what I would do. Both parties in the affair are responsible for the affair. However, HE is the only one who is responsible to answer to his wife for the affair.

Friends dont hide the truth of things that matter to them. They dont defend the friend who perpetrates an immorality while sit by and let the friend who is the victim of the immorality stays ignorant. Are you really being a friend by letting one commit immorality and letting the innocent other be a victim? Truth is always everyones moral obligation... we all should be held accountable to the truth, even if its none of our business... because truth and morality is everyones business. I think the MM should stop, of course... but I also firmly believe the wife has every right to know. If my wife was cheating, I wouldnt want to be kept in the dark about it. Yes, the truth being known might end a marriage, but that is my decision, not my wifes (she already made her decision)and certainly not yours. If I ever found out you were protecting the perpetrator at my expense, you would lose me as a friend. That would have been your decision, though, as it is betrayal. Being betrayed by your wife is hardly different than being betrayed by someone who calls you a friend. No friend, no human being, should ask or expect you to keep a secret of immorality. And your own ethical standards should supersede your friendship. Your personality, and your ethics, is in part what makes people worth friendship in the first place. If you compromise who you are, then what are you? Why would you make a good friend and are you worth friendship? Asking you to violate your ethics is no different than asking you to change, habits, hobbies, personality. That is an absurd request from any would-be friend. If someone wants friendship then they should embrace you for who you are, as is, letting you keep your morality in tact. If they have a secret they dont want getting out, then they shouldnt let you know about it.

stick an anonymous note in the wife's purse when no one is paying attention, then let them handle the rest.