My friend is cutting, what should I do?

In my opinion, you should be there for them. Not shame them about it, it can be very difficult to stop if your friend is addicted to it, but just being as understanding as you can, and making it very clear that you will be there for them. If the cutting is really harmful, in the sense of that the wounds would need tending, but maybe your friend is not going to the hospital in fear of getting shamed or caught, you should encourage them to go get treated, and in extreme cases or if you feel like you can't deal with the situation alone, you could tell someone. If your friend is underage, maybe their parents, or someone who could help them.

You should not pressure them into stopping cutting. I bet they know that it's bad and that they should not do it and you telling them again will not change anything. If you wanna help try give them the feeling of not being alone. But remember it's not your responsibility that they feel better. If you think it's getting out of hands try talking to your parents, or an teacher. If you are older talk to her directly and suggest her to get help and let her help herself. That the best thing you can do. Listen to her understand her. Take care!