As a teenager myself, here is my advice. Just be nice. Give her the feeling that you are always there for her. Even to get her out of a mess she has put herself into.
When she decides to change and come back, shell know there is SOMEONE there to help her.
NOBODY can change her. Only she herself has to want to change in order for it to happen. So if she doesn’t want it, don`t bother pushing too hard.