Damp, for the grace of our holy Lord, Jesus Christ, the son of our Father, let her drink his holy blood in the sacrament of communion.
Try to convince her to throw away the flask and never drink again. If she doesn't, fine, that's her prerogative. Don't be a little snitch, because you will be hated by her regardless of her parent's reaction. And you can't fix that.
When I found a bag of weed, some paper, and a lighter on the back bumper of my trailer with my friend's brother, I told him to keep it a secret and just threw it away. He told his father, but his father just asked if I threw it away and I said yes. I knew it was one of my sisters or some wierdo, but I never told any of the units because I knew stuff would happen.
Regardless, weed isn't really bad for you in the sense, but you shouldn't be smoking it when you are 14/16, because that shows you have interest in drugs, hence why they call it a gateway drug.
Just don't start stuff Damp. Just try to convince her that what she is doing is bad without a middleman. Wine isn't even bad, first off. And being young, she probably has very cheap wine.