People cheat for different reasons.
Bingo!
Was his cheating a one-night event? Or an on-going affair? Sometimes the reasons for cheating are as simple as some mis-guided ego drive that feeds on the attention, or an internal need to do something "wrong" and dirty. More often, it's because something is missing (or is perceived to be missing) in the primary relationship.
Forgiving, or breaking up is a decision for the two of you, and not for your friends or anyone else. If you believe the relationship is worth saving and you are both willing to put in the effort to save it, I would recommend reading some of the books by Dr. Willard harley. He has guided a lot of couples through infidelity. He also has a website: http://www.marriagebuilders.com.
Good luck.