This is quite the medical issue; however, since you asked what you, a parent, can specifically do....
Recognize the gravity of the crime
A lot of people miss this part. Recognize and believe that what happened to her was a serious crime; a violation of her being. There is no "getting over it." There is only accepting it and managing it.
Understand why she is doing this
It's difficult to understand, but cutting yourself is a great pain reliever. It's a way of releasing the pain they are feeling on the "inside." For sexual assault victims, it's also a way of re-asserting control back on their body -- something that is taken away from them.
Cutting is fairly serious and it should be dealt with. However, baby steps. Asking her to stop cutting herself will result in lashing out -- you are taking away her right to her own body. You have to lead her to the desired behaviour; not demand it. This can be something like talking about cutting; asking her what kind of tools she is using. You can talk about the safety of it -- remind her to use clean tools. You can ask that she give you a call after she is cutting, since you are concerned if she is OK or not.
The goal is to make her talk about what she is feeling. Make her realize that there are alternatives (such as talking, therapy, etc.). Make sure that she knows when she is ready, she can go to therapy. Help her find a psychiatrist -- they are not all equal. You want someone who she will be comfortable talking to.
Also, love. TLC is not just for babies. It's for adults too. Touch, love, and care.