Im kind of worried, help!?

She's only 17 but I seriously think she's an alcoholic! I mean, we all drink sometimes but she always does. She already had an underage drinking fine, and when we go to parties shes the girl that you will look over and see her half naked dancing on a table. hahaa, its funny sometimes but the other day I went to her house and she was drunk all by herself and i was like girl, who gets drunk by themselves? shes ALWAYS drinking. She took a water bottle filled with vodka to school before, thank god she didnt get caught. what should i do?!