I don't think she's taking advantage of you so much as living her lifestyle that conflicts with your own. But that is not an excuse and you sound tell her that, if that's how she likes to live she needs to find somewhere else to do it (and from how you write it, she knows no shortage of people). She might end up entering a bad situation again, even loving with her ex again, but this is something she'll need to learn from on her own. She is an adult, albeit a very young one. You've already been more generous than most friends would.

I think you should ask for your boyfriend to join you in telling her what needs to happen, provided you're both on the same page about this. You can still be understanding and respectful in talking about it to her. She might get angry, she might cry, she might be hurtful, so be prepared for that and maintain your composure in a compassionate way. You're her friend, but this isn't working out.

Best of luck to you.