You're in trouble, that's for sure.
If you are considering ending the relationship (and you should at least consider it), I see no reason why you shouldn't confront her directly about this other guy. In fact, even if you stay w/her you should still confront her. Ask her why she needs to spend so much time w/him, what's up, etc. And listen to her answers, especially if they at all concern needs of hers that you aren't meeting.
As another poster (Tedel) mentioned recently, cheating is horrible, but it usually happens b/c there are unmet needs on both sides. Maybe you ignore her feelings, maybe she doesn't feel trusted (duh! she's a cheater), mbe you communicate really badly. Of course, mbe she is just a cheating attention whore. Only you know for sure.
If your relationship is going to tank anyway, I see no reason not to confront her. Keep some dignity & try to salvage what you can from this as a learning experience for yourself.
Good luck.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh