I am sorry that your friend is having such a hard time,and also that you are so worried about her.
When I talked to my friends, it was always helpful to here that they cared, that what I was doing was effecting the people around me an not just myself. For many self-harmers, there is all ready a sense of guilt and shame over their actions, so it may help to talk to her about it not being a weakness or an inherent problem with her, rather we all have our demons and this is hers. Talk to her about asking for help, about how brave that move can be, and how no one will blame her, they just want her to get better. Maybe try to figure out the reason behind her cutting, that often is the first step to recovery. There is a great thread on here about different reasons that people cut and self-harm, if you would like to check that out.
The most important thing is for her to know that you care, that you support her, and that she is a good, worthy person (which I'm sure she is to have a friend as supporting as you.

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"I am on my way, but I don't know exactly where it is I am going. But that's why I'm writing: to find out."- Julia Alvarez
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