my friend might be cutting herself?

[Niall]

well my friend has been quite depressed lately and won 'tell me why
a while back she told us about how she cut herself a few times in the past and my other friend told her to promise she wouldn't do it again, which she didn't
and right after she put up a comment on twitter saying she won't make promises she wont keep
well just now tweeted saying that she's glad she didn't promise and thank God nobody will know what she's talking about and i think she might be doing it.
what should i do
and should i tell the other friend who knew about the first time about what she said?
she won't tell me whats wrong and the thought of her doing it is actually bringing tears to my eyes
please help