Being 15, I'm gonna say be nice about it, but tell them it's not the best idea at the moment.
Pay attention to who she hangs out with, who her friends are, who introduced her to it... Blablabla.
Ask if she actually knows what it'll do to you/what it CAN do to you, and if you know someone who was influenced by it when they were that age, get them to talk to her in a friendly-ish way.
Both of those could have already been mentioned, but the blackberry's browser is too slow, I skipped to the bottom