Would you tell or not?

I was chatting with my son today, and his friend, and my son was relating how (fictional names) his buddy called Mark had a girlfriend called Jill. He was primarily Marks friend, and really liked him as a friend. But one night in a tipsy state, Jill told my son she was having an affair.

My son then told his friend Mark. The friend that was with us, said that is a definite no, no. I tended to agree. But the more I thought of it, the more I decided that it was the right thing to do. Since as my son put it, Jill was basically making a fool out of Mark, and as his friend it was his duty to put a stop to that.

How would you handle this, if your best friends girlfriend confided this to you? Would you feel it better to save them the time wasted on this person, and tell them? Or would you let life take its course? If you decided not to tell your friend, what would be our thinking behind it?