Hello and welcome. As a mom of two daughters, my heart just breaks for you. I am so so sorry you are going through all of this.
I would do a couple of things. One is immediately report the boy for purchasing her alcohol. I second inpatient treatment if possible. I would also go and have her drug tested for everything. That way you truly know what you are dealing with. From the sound of her angry, aggressive behavior could be possible she has been mixing benzos or some other drug with the alcohol. She also just may get that way when she drinks.
I firmly believe kids that age drink due to peer pressure and the people they associate with, at least that's how I was. I drank a lot at that age. It never progressed into alcoholism but looking back it was very clearly binge drinking.
Tight hugs to you. I hope you can reach her and that she will tell you the truth. I have said and heard on here several times, prepare for the worst, hope for the best.
God Bless.