How to get your ex to stop cheating on other girls?

My ex and I have had troubles. He cheated on me for several girls. Then he's now dating this one girl and I've caught him cheating on her. She knows he has cheated and she talked with him and he said he wouldn't cheat again, yet he's still cheating on her. I am his friend and he has tried to cheat on her for me several times. He has confessed he's a w*ore but he will not stop. His girlfriend doesn't seem to do anything about it since me and my friend have told her recently when he wasn't at school. How do you get a w*ore to stop cheating on their girlfriends?