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.
You mean your ex-girlfriend cheated on you with your ex-best friend. Avoid all contact with them. Find better people to be in your life.