Hi susieg,
I'm so sorry this is all going on. Finding out about an affair (of sorts) so far in the future is a tough thing.
As for telling the girl's bf, do you know him? How would you plan on telling him? Do you think he would want to know? Or maybe... would it be possible to tell a friend of his so you're not the one breaking the bad news (in the case that you don't know him well)?
As for your bf, I don't think it's ever excusable, personally. Especially if it happened more than once. The fact that someone else had to tell you and he did not also speaks about his character. How are you to know that's the only time? Is he only begging for forgiveness because he got caught or because he is actually sorry for what he did and will never do it again? And, most importantly, do you think you can learn to trust him again and not fall into the mistrust?
It sounds like you have a pretty good head on your shoulders. Unfortunately there isn't really a "right" answer in this situation. Good luck! :)
Love 
