What should i do to help ?

you should talk to her be with her make her feel that there is somebody with her !! ask her why is she so depressed do everything but if it didn't work tell ur friends parents and remember it is what best for her so do not feel that u are betraying ur friend but telling her parents will help her not to get in drugs and stuff this will affect her whole future and may be cause for her death !! but try to talk to her first see whats the problem!!