Sounds like you should forget her for now as a band member. She has already proven undependable. If you choose to try and offer her support I commend that but business is business and the place you book a gig doesn't want to hear your guitarist/singer didn't show....again. Next thing you know, no more bookings. When I was younger and still drinking heavy many people tried to help and I was even court ordered into re-hab. I didn't quit until I wanted to and then did that myself. That is how that works, all the support in the world won't cure her, she has to want it and I mean really want it. She also has to have the will power to do it.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!