Finding out that your girlfriend is cheating on you is a real kick in the nards. It's even more devastating when you discover that the other guy
Should I tell my best friend that his girlfriend is cheating on him? Tell her you know and she can either tell him on her own or you will. Variation on #1: Have evidence, like pictures or something solid. Figure out what's going on and see if you can predict where she will be with this other person.