Bummer man. Did you check to see if all small valuables in your home are still there? Had a friend in high school that had the "party house" one weekend his parents were out of town. Turns out his Mom's engagement ring and a pair of expensive earrings were taken as the totally awesome cool party went on. Some weakly-connected guests thought nothing of his and his parents belongings and nabbed the expensive goodies. I'd double-check the house before you let this one settle.

Also, as far as telling the Mom, you may want to consider some CYA (cover your arse) in that the Mom could come back to you for withholding information regarding her minor-aged daughter being in the presence of illicit drugs and alcohol at your house. If she was dumb enough to leave your house a mess, this kid may be dumb enough to have info of that 'sweet party' at the house she was watching, get back to her Mom.

I'm mainly speaking as a middle school educator on this one. You aren't directly responsible for the well being of this young girl, as you were hundreds of miles away and didn't contribute in any way, but if the Mom wanted to really push things, she could attempt to shirk her responsibilities to monitor her daughter's activities and blame you for not filling her in. I'm not sure what the consequences for not letting her know could be. Maybe nothing other than an angry parent. Or, she may just say thanks for your help in helping keep her kid from getting hurt.