It's not your place to tell his girlfriend that he cheated on her, as tempting as that may be. Instead, I would talk to your friend first. Let him know you believe what he did was wrong and that he owes it to his girlfriend to be honest with her. Tell him this is important to the health of your friendship. If he is regretful of his behavior and values your friendship, then it may be fine to stay friends with him. But if not, you may need to distance yourself from him.