Your GF obviously has some sort of feelings for this guy to keep him around. It's better you found out now then a year or two down the road. I know it hurts bad but sadly it needed to come out. Take it from me I've been cheated on and my GF at the time got pregnant in the process but man am I ever glad I had a friend tail her for a few days and brought me the dreadful news/evidence that she was cheating. ( I had reasonable grounds to "spy") I'm not controlling or anything like that trust me I'm very trusting but it was a last resort for me in order to move forward. When I confronted her about the other guy she was more angry because she got caught and in fact broke up with me before I had a chance to dump her weird eh? If you cannot be open and honest with your partner then what's the point?