W.G.
C.
You sound like a very caring and concerned friend to this grandparent who like many others finds herself in the middle of parenting grandchildren today. She's lucky to have you in her life to support her. So much has changed since this woman parented her own child/children; the landscape which today's teens navigate is very difficult terrain full of lots of cliffs over which many fall.
From the brief description you've given, my immedicate response is to question whether or not this 15 y.o. has become involved in drugs or alcohol. The behavior seems classic. I'd recommend talking to the girl, her therapist and doing drug testing which can be done through the teen's physcian or there are many over the counter urine drug tests. In talking with the teen, she will likely deny any drug use. This is a "normal" response from most users. Test anyway. This young woman's life may be at stake and now is the time to take action while she's a minor and the grandmother can exercise control and get the treatment she may need. The sooner treatment is begun the better.
If treatment is needed, I'd recommend a family program so that the grandmother and any involved extended family members can attend so that they understand the nature of the problem and understand their role in supporting and not enabling this young woman who may be struggling to survive. If possible a program that focuses on attachment needs and uses or is familiar with Emotionally Focused Therapy would also be good.
I hope I'm wrong about my reaction; however, if I'm not, though this may be difficult to hear and to follow through with, it may make a real difference in their lives.
W.