Teens can be terribly protective of their A parents and actually take a parental role towards their own parent (I saw it happen with one of my kids and her A father).
Because of that, an internet forum probably felt like a "safe" place for her to seek help. I think I would probably keep talking to her, and try to slowly convince her that this is a problem that is too big for her to handle, and that she does need to find outside help -- for herself, not for her mother; that's not her responsibility.
Alateen is a great start, but knowing first-hand how difficult it is for an adult to get over the resistance of going to an Al-Anon meeting, I can imagine it may feel terribly difficult for a teen.
You could refer her here -- we've had teens with alcoholic parents post here before.
I'm guessing getting her to talk to a school counselor would be hard?