Having brought up tow daughters I can empathise with your position @booboo
We had a very similar situation when my eldest told me (in confidence) that a friend of hers (and the daughter of a good friend of mine) was smoking dope at the ripe old age of 10. What to do?
First was the discussion with Ms Cato explaining that we cannot let her friend potentially harm herself without helping (agreement), then discussing how best to help her. Talk to her, tell the school, tell her parents etc (eventual agreement that telling her parents would be the least bad option). So in the end I grassed her up (pun intended) which caused some problems between them but I had readied Ms Cato for the fallout and importantly I had not betrayed her trust.
Good luck with it all. Loads more coming your way if my experience is anything to go by.