Do you tell your best friend that your other best friend is cheating on him?

My wife and I have another couple that we consider to be our best friends. Recently we were confronted with the news that she is cheating on him as a result of a long standing bitterness over a past mistake of him cheating on her. He cheated once and realizes his mistake and has moved on without ever doing it again. She is has been talking to another man for the last five years (since her husbands slip up) and recently has slept with him several times. She is in limbo and waffles back and forth between staying with her husband of 10 years and two children or leaving that for this other man, who is also married. The question is...do I tell him she is cheating or do I just let fate run its course, and help my friend pick up the pieces when he does find out, because I think eventually he will. Is it my place to tell, also I'd be breaking her confidence in telling. I don't want to leave a clue so he accidentally finds out, I either want to tell him or not, which one and why.