So it's obvious that she has feelings for this guy, and I think she feels "powerless" to him in this situation. And I think that maybe her ideas of relationships are so screwed up from possibly growing up or a bad history that she thinks this is normal.
I think she needs to be on her own for a while and figure out what she wants. She sounds like she has alot of growing up to do, especially for wanting to date a guy more because everybody else told her not to. When she does grow up, she'll look at this and say "What the hell was I thinking?" Her self worth has to be completely non existent, anybody in the right mind that gets treated like this should know better. How many times will it happen before she finally gets it? He's never going to learn if he always has her to come back to. You are powerless to how you feel, but you aren't powerless to what you do. You make your own conscious decisions and she has to acknowledge that. It's her fault to stay in this. She has to accept responsibility for her actions and find ways to be happy on her own so she can have some self worth and not tolerate this bullshit.
I emphasize that she be on her own, because it doesn't sound like she has been. She's tried to date right after they broke up and with her still caring about him, it completely fizzled. She's alone again so it's back to him. She has to understand that she shouldn't have to need somebody in her life to be happy. She's gotta take the reins herself and find that happiness on her own before she can find somebody else and increase that happiness.
Of course, if she doesn't want to listen, she isn't going to hear. Hopefully you can sit her down and just explain some of the things I mentioned above. Ask her if she's happy? Does she know why she isn't? She has to kick this guy and get him out of her system, instead of doing the drug even though we know it;s bad for us because we can't handle being alone.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.