B.P.
My daughter is now 25, but when something similar happened when she was a teenager, I was very firm with her that she was to have no contact with this girl until she got her life back together. I advised her to mention to the girl at school that I was suspecting she was using drugs and was prepared to call her mother at any given time and have a discussion about it. I, like you, felt that the mother might know something, but was in major denial. Luckily, my daughter and this girl were not best friends and it was not a huge issue for us, but it turned out I was right and the girl was using drugs and alcohol. She became quite a handful throughout high school for her parents and she lost several more friends.
My advice for you is to "go with your gut". Talk to your child and discuss the good, the bad and the ugly about this situation. Discuss with her how not to be a rescuer in this situation. The boy is making a choice to use and it is not up to her to save him. She can give him information on getting clean, and leave it at that. Have her tell him that you are prepared to call his mother and have a deeper conversation if he ever acts this way again around you or your daughter. He will get the picture. Good luck!