Same rules apply if she's self-harming. You can never force someone to get help.
Self-injury isn't the end of the world. It's a coping mechanism that indicates she's having some troubles, but she's trying to deal with them as best she can. Self-injury is very different from being suicidal, I'd posit that it's pretty much the opposite. It's someone trying their best to live through something difficult. Now, of course you want to help her get past whatever the cause of her grief is. That could be something as big as sexual abuse or something as seemingly insignificant as feeling on the outside at school. But still, you can't force her to talk to someone about it.
You can try talking about it with her yourself. But don't be confrontational or judgmental about it. And don't make it imperative for her to stop injuring herself either. If she's gotten to the point where she's hurting herself, she's tried other coping mechanisms that haven't worked. She needs to self-injure right now. It's what she is using to keep herself sane. But if you are supportive and not adversarial, there is no reason why you shouldn't be able to talk about it with her yourself. Bring up some options - therapy being the one you're looking at obviously - but if she does identify anything as her particular cause of stress, maybe come up with some alternate solutions. For me, what would have helped was changing schools when I was in Grade 8, or even just some more positive support from my mother. I needed her to spend time with me, not fighting with me. I'm sure I was a witch to be around some days, but I needed her unconditional love and support. That would have made all the difference in the world to me.
Be nicer. That may be all she needs. If she feels alone (and almost all self-injurers do), that might make it a bit easier for her. Enough to tackle some of her other demons.
Rachael