you should try and maybe go to some addiction recovery meetings in your state to help you with ideas on helping her. Possibly even go with her to some o these meetings. try and keep her focused and tell her that there are probably underlying issues with her taking drugs and thatss the stuff she may have to deal with in order to "fell happy again" drug free. She needs to stay strong and focused maybe a doctor might be able to help her with some sort of medication to help her in the meantime and talking to a psychologist could also do her some good if she's willing to go & talk about it.
She's a brave girl for quitting, it's hard work!! I really hope she stays clean but rememeber that it's not really in your hands but you can try and be as supportive as you possibly can.