hi, i am only pregnant, so i havnt been in this situation with you. But when i was younger, my brother would tell me things that were going on in his life. And my dad would want to know. We worked out, that if there was a bit of issue, but i felt it didnt need to be told, i would just say 'things are going on, but dont worry'.
maybe you could do that kind of communication with the mother? my dad didnt like not knowing, but he trusted my judgement for when to worry ect.

i hope you can help this girl

Really if she is going to do something and it is dangerous to either herself or another person you need to tell her mum , after all she has the right to know , i think maybe you should tell the mum the important bits , by the sounds of it as much as your tryin to stop her rebelling she may do that by herself , but i would tell the girl  what your going to tell her mother first .

i wish u all the best with it.

thats a fine line and you need to decide which part your going to stand on, being her friend is good but you also know all about their hormones at that age, if you think she is going to do something dangerous then you need to tell her mum, you can be a good listener to this girl but dont give her any advice unless you think about it first!!

Lett her mum know that you are a shoulder for her daughter to lean on but will tell her if there is anything serious going on..
Maybe you could explain to the girl how you would feel if your child did these things to you.
and maybe you could talk to her mum and maybe she will let you take the girl out places with you so she can have some freedom but still be supervised.

Good luck and i hope things work out for the best.