How can I help this girl?

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So there's a girl who's in my school in the year below me, and she's made a twitter account posting lots of inappropriate nudes (which have her face in so you can tell it's her). She's also putting tweets such as "I do free meet ups, if you can make it to me I'll **** you" etc even though she's only 14/15. (She pretends she's 19 on the account).
A few people in my year and her year have found out about it but she doesn't know. In school she's so quiet and shy.
Because I found out from someone who already knew, I know that in a matter of a few week everyone in her year and mine will know.
Although she shouldn't be doing it, I really feel sorry for her and want to tell her to take the account down before everyone finds out, but how can I tell her? I have her on Facebook but what can I say? I want to help her out before everyone sees