It's good that you are reaching out for ways to help your friend. Its really scary when someone tells you they are cutting themselves or thinking about it. It sounds like she is really stressed or going through a tough situation and is trying to get some help.
Talk to her about how concerned you are. Tell her that no matter what she's dealing with, cutting isn't a good way to cope with that pain. Let her know that you have cut in the past, and that you regret it and that you don't want her to go down a similar path as yourself. Sometimes it can help to see a "first-hand experience" to get a different perspective before you do something.
It would probably help the most to tell her that you want to stop cutting also. If she sees that you are willing to stop cutting (that it is something that doesn't help, isn't good for you, and is a bad path to go down) she will be more likely to not cut herself. You can even ask her to help you quit. Talk to her about all the negative sides of cutting, and encourage her to deal with her pain in a healthier and safer way. Maybe you guys can do some "coping skills" together. Call each other when you feel like cutting, chat online. Go for a walk. Text her a favorite song that you're listening to (one that is upbeat and not sad) when you feel like cutting and she can do the same. Journal. Write letters to each other about how you're feeling, then give the letters to each other the next day. You could even encourage your friend to email us or chat with us online when she feels like cutting in order to vent and get some support, rather than hurt herself.
Do you think any of those suggestions will help? Talk to her about how you don't want her to start cutting because you know first-hand how dangerous it can be. Let us know if you need any more help or how the conversation goes. Good luck!