How can I make my friend see she's making a mistake?

So my friend is kind of a sex-o-holic. Has been for a while now. This guy we've both known for almost a year through church (yeah, i know) likes her. She seems to have a knack of bringing different guys to church each week, and we never see them again after. The guy sees them all, but ignores them. Lately, they've been on and off talking. The guy is really really sweet, and HAS a future. (full scholarship to a great school, a job, nice body, smart). Yesterday, some things occur ed where he was hyper and talking a lot. He was talking to his ex. Nothing bad, but through the conversation, you could tell they were close. (Well, my friend and the girl hate each other.) They have history. So the next day, we all hung out, and she talks to him about him talking to his ex. She asked if he was playing her, and he so no, and she just wanted to be sure he was there for her. They made up, which I didn't see the problem, except for that she got jealous. Her and the guy hung out the rest of the day. Well, until some other guy (she doesn't know him) came and she was obviously flirting with him. He doesn't get mad, but ticked. They go into the house, and I over here him say, "how are you going to tell me to stop this, if you keep doing this?" and her say "oh, I just mean nobody close to me." She kisses him and leaves with the guy to a movie. He ends up leaving the house after she leaves with the guy. You could see on his face that he's about to give up on her. She came back saying she wanted him later, and the movie date was a make out fest and nothing more. I think she's done too much to my friend, and can't really give too much advice other than stop looking like a " you know what" around him. But I can't say that to her. What can I say? Any advice?