Before doing anything of that sort, you need to advise her first and make her aware of the serious consequences of her actions; you should also let her know that you are going to bring it to the attention of her parents is he does not stop. By doing this you are not incurring any sin. It is not different from trying to save someone who is drowning. You wouldn't simply watch a person drown; rather you would take all the necessary steps you can muster to save him or her.
Since the spiritual is far more serious than physical death, saving the soul should have greater priority. So, never mind what she will think of you, you cannot simply let her drown and perish.
I pray to Allah to guide us to the truth, guide others through us and make us instruments of guidance.