Cutters are often very depressed, or anxious. Cutting is a form of meditation that actually helps them feel better. They focus their mind on the cut and the sensation, and it distracts them from the horrible pain going on in their mind.
(At least this is my personal theory. I asked my psychiatrist and she said no one really knows or fully understands it.)
Fighting cutting is like fighting smoke. It's really just a secondary side effect of being serverely clinically depressed or anxious.
It's good you're her friend. That probably helps a lot.
My approach is to accept the cutting and focus on giving them hope that the underlying problem can often be successfully treated. Medication often works, though one may need to try a whole bunch of different medications before finding the one that works for them. (You'll know when you find it.) There are numerous anti-depressant medications to try. It's wonderful when you find the right one (like I eventually did).
Ice meditation may also substitute for cutting meditation. Get an ice cube and put it on your body, and focus all your attention on that ice cube, and the feeling it produces, and watch it melt.
Other things that can help combat depression is Mindfulness Meditation, where we practice meditating without ice or cutting. We just practice focusing our mind. (We want to activate the Parasympathetic Nervous System.) Also Therapeutic Yoga, Tai-Chi, Qi-Gong, are mindfulness motion exercises. Exercise is also good.
Having a supportive group of friends helps. Being her friend can make a huge difference and help her get through this very rough time.
My personal favorite though is finding the right medication. Once the right medication is found, the brain can then heal itself. Healing continues for a long time, like a year, and things just slowly get better and better as the brain heals itself.
(I was just talking to my psychiatrist today about the hippocampus, which stress can eat away at like acid, and brain scans show it's smaller in depressed people, so long term stress can lead to depression. However it can also be reversed. Meditation can increase the size of the hippocampus. I'm guessing that's also what happens when the right medication is found. It takes time for these changes, but it can be achieved.
Recommend friend see a doctor. Any doctor. A general purpose doctor is a good place to start. Or you can go straight to the specialist, which is a psychiatrist doctor who can prescribe the medications to try. (Psychiatrist = doctor, Psychologist = talk therapist. Talking with someone periodically can also be helpful.)
And having you as a friend is likely very helpful too.
The cutting will stop on its own once the underlying depression is treated. If you have to tell someone who's not a doctor or psychiatrist or therapist you can omit the cutting part and just say she's severely depressed or anxious (ask her which it is. It may be both.) Most laypeople don't understand cutting and they just freak out and focus on stopping the cutting. You have to imagine feeling so bad that cutting yourself actually feels good. (And no they do not do it for attention. They usually try to hide the scars.)
The good news is the underlying problem is treatable with mediation (which may sound weird at first. The pills don't make you happy, instead they fix the underlying problem so you can feel happy again, instead of constant mental angony. They often take 2 weeks to start working.) Give her lots of hope that the underlying problem can be cured so if she can hang in there and get to a doctor who understands this stuff she'll likely have a good happy life once this problem is fixed. (I survived by being given a lot of hope that the underlying problem could be treated if I just stuck it out and kept going back to the doctor and trying different meds.)