Kitten, I have no doubt she provoked him tremendously. I just don't want to see her dad in trouble on top of your worries about her. It's not worth it. You are on top of it (a very good thing). You know this is in the family and I'm sure you know that substance abusers of all stripes QUIT their abusing every single day. This is not hopeless. She is only fifteen and you have a few years to try to help her.
In a way, I feel blessed (if one can call substance abuse a blessing) that my daughter started so young and that we caught it so early. We were able to influence her more than if she had already been legally an adult. We did all we could, including letting her go on probation twice. We wanted to scare her straight. I don't know if we did. Honestly, this is such a slippery slope. But this was a kid who snorted ADHD drugs (yes, you can if you crush with pillcrusher), used cocaine, psychedelics, tried heroin a few times, and loved speed/meth and she did quit young too...nineteen. Maybe if they start young and we catch it and they know we care and want to help, they are less apt to keep doing it as they get older and can stop more easily in the resiliency of youth.
I do think family counseling may make a difference because you can all learn to interact in a positive way and maybe she will feel safer with somebody who is not in the family helping all of you, just not her one'on-one. She is probably pretty scared at her habit. My daughter admits she was, but she was even more afraid to lose her loser friends. She was/is very shy and drugs made her outgoing and popular and once she lost weight by the meth diet (tongue in cheek) the boys just chased her like crazy. She had to get over that and she did.
You or a professional can drug test her to see if she has a lot of alcohol in her system too. Not sure where this fits in. I just am hoping your young daughter decides to give it up before it gets way out of hand. She is still capable of living a full and wonderful and sober life.
Wishing you all the best. Keep us posted. We are here, cheering for you.