Agree with other posts:
They can't treat her without parental consent because she's a minor. If you're really that worried about her, talk to the authorities. Contact child services. If she does have a problem and her mother's "medicine" isn't helping, she's neglecting the girl. Child services needs to know it.
Though remember that if you get involved, it will more than likely mean the end of your friendship/contact with her. Her mother can and will forbid her to see you, and the girl will probably, based on what you've said, listen to her mother. This is all in case she gets to stay with her mother. You also have to think about the fact that child services could deem the mother unfit and remove the girl.
All I'm saying is you need to be sure, before you get in the middle.