To the above poster, unless there is proof he's cheating now, it isn't helpful to the friend to bring up what happened in the past. My sense was that the op just wanted to relieve her conscience, not look out for her friend.

In the place of the friend, I might want to know too just so I could cut them both out of my life. But it would hurt horribly. So in fact if it didn't affect my relationship, I actually wouldn't want to know.