The answer for most is you don't, but you learn to live with it, and then move on. Unlikely you would trust him again, and the only reason you know about it someone else told you. Then he had no choice to confess, which is bad enough. So stark choice, do you forgive, but live with this knowledge, or move on. Personally i would have to move on because i doubt i would trust that person again. As everyone here will say i'm sure, a very very difficult subject, choice, and one only you can make it.