How can i convince my bestfriend not to get back with his crazy ex girlfriend?

They dated for two years, she is very immature and lied to him about some things that he later found out about. They were about to be engaged but she would often do something really stupid that would push him to the brink and he would threaten to break up with her although he didn't really mean it. So she told him that she didn't think they could ever find happiness being together and that if he really loved her he wouldn't threaten to break up after he said he wouldn't. She refused to give him another chance and they broke up. The thing is, he has done so much for this girl, he was always there for her, gave her his extra credit card in case she ever needed something, and bought her a $2,000 engagement ring but she's never there for him. I don't trust this girl at all, she seems so fake to me, but I think my friend still has feelings for her and would get sucked back into a relationship with her if she wanted to make things right. He seems confused, he's been in a few relationships where he thinks the girl is the one but he always ends up getting screwed over.