How do I stop a girl from falling for a guy who doesn't deserve her?

A little while ago I heard a friend of mine recently got a job at a movie theater with the help of my former roommates. One of them however was really obnoxious and arrogant beyond belief and I never liked him at all. Recently, he's been hitting on her and it creeps her out but I have this horrible feeling she's going to warm up and probably come to love him. All I want is for her to be happy, no matter who she's with, but he's just so wrong for her it's not even funny. I mean she's this really sweet, wonderful girl and he's the most conceited asshole in the world. I don't know about you, but i'm tired of seeing nice girls fall in love with douchebags. Also I'm not exactly the best matchmaker. I don't know anybody else who'd date her and I know for a fact she'd never want to do anything romantic with me. I'm not saying that I decide who should date who, but I care for this girl even though she's come to somewhat resent me in recent years. What would you do in the same situation?

P.S.- I have considered the possibility that this all a paranoid delusion and that I'm only feeling this way out of contempt for him, but you never know.