wow...sounds like you've got a really serious situation here...
slugboy i already commented you but i'll add to it a bit on here...
she needs someone who understands her and that she can trust...i've been in this situation...on boths sides (but i never cut...ever) trust is a massive issue.
she needs to be able to talk to you without you judging her
never treat her like a child or tell her that its stupid
find the source of the problem (stress, abuse, etc?) and try and help with that
also tell her there is much better ways to get rid of the stress or depression....
help her start an "emergency box" tell her to fill it with everything she loves or that makes her happy... chocolate, candy, photos, jokes...maybe sometimes slip a letter in there for her...then whenever she feels like cutting make her go to that box and it should make her feel better...
i have more... but that's enough for now....