Her friends are bad influences, so you introduce her to a drug dealer. Awesome....
Anyway, she needs to get out of that town. Period. As long as there are bad influences that she is drawn to, things won't get better. If fact, I doubt she wants to get better, she just doesn't want to have to starve.
Unfortunately, by this time, her life is ruined. It's not like she's just had one or two unfortunate circumstances that brought her to an all-time low. She has had an entire life of fucked up shit that makes this situation inevitable, and probably not all that strange for her. Yeah, it sucks, but "helping" her only accommodates this fucked-up lifestyle she has become accustomed to.
Cut your ties. Explain to her that your GF is insanely jealous, and you have to quit seeing her. Then tell your mom that she has to kick her out. Eventually she will move on to the next person she can drag into her shit.
