Moral Quandary

so here's a moral question for you.  if you think this is about you, it might just be. 

what if someone you knew was having an affair? and you know. but their marriage partner does not. you're very good friends with both partners in the marriage. veryveryvery good friends.

the thing is, the person having the affair doesn't know that you know. you know because the person she/he's sleeping with is also a veryveryvery good friend and has told you.

what do you do???