There are two sides to this.

1. you've given her her own space, her own privacy and you invaded it by checking in her "place", I assume you didn't ask permission to enter

2. she is doing something illegal and quite possibly something that will damage her brain function.


For point 1, that is something you will have to deal with, both the betrayal of you being somewhere you shouldn't and of her having something she shouldn't have

For point 2, I'd contact drug arm (or some such organisation), get some information on drug use and developing brains. Give this information to her. Talk to her calmly, not only from the point of it being illegal, that possession & use of it can lead to convictions, but of the damage it can do to her body.