Do everything you can to get him away from her. He is a kid at 15- that gives you control to a great degree. He will probably start doing drugs again if he is with a drug dealing person, and in that environment. DO not put up with it.
Trust me I let stuff like that go, and my son is now 25 and shooting oxy and overdosed recently again. If only I could have went back to when he was under my rule at 15.
His life could depend on the choices you make now!
"I found this out from reading his private FB messages. I know, I violated his privacy. I can't use the information I just got, obviously"
That is Not an evasion!!! good for you!
Just please do whatever you can, now! Whatever it takes!