My best friend is being dragged along by a scumbag.

Sarai • Cool and confident, with hair like Django. ➰
My best friend of 6 years has been dating this LOSER for 2 years. I'm the more level headed, realistic one and she is the hopeless romantic, lead-with-your-heart type. 
He cheated on her and had a mini-relationship with another girl, all without her knowing. He's selfish, greedy, whiney, negative, and a user. He is just all round crappy. She loves him so much, and is so afraid of being alone that all he has to do is shed a few crocodile tears and say sorry and they're back. 
I've been honest with her so she knows where I stand, and she knows exactly where this is heading. I just want to know how can I be a good supportive friend without being harsh and angry at her stupidity?
Feel free to share experiences with supportIng naïve friends!