How to I get my friend to stop self harming without telling on her?

Remind her of things she has forgotten. Randomly call her to tell her you care. Accompany her on nights when the world feels like it's been turned upside down. Spend time with her whether it's watching movies or eating or just staring into space. Never forget to tell her of her importance in this world. Make her feel without letting a cut slice through her skin by making her feel as if the emptiness was a temporary thing that'll leave soon enough.

I think there are several ways to help her. One could be helping her find alternative ways to distract herself from self-harming. I personally like this one because it has helped me before: http://pretentiousfacade.wordpress.com/2013/02/18/alternatives-a-good-read/

No one can force someone to stop harming. Even in the safest places - like inpatient wards - people can harm themselves. However, there are many ways you can support your friend. You might let her know you care, offer to talk with her or keep her company if she wants that and you are comfortable with that, and you can help her find resources. There are online sites, lists of distractions, and many other types of coping skills. I would advise looking into a DBT style workbook or information. However, if your friend is at risk of killing herself (either suicidal, or harming in ways that require medical attention) and no one knows, it might be time to heavily encourage her to talk with a medical professional and/or tell her parents (depending on her age).

When I myself self harmed the worst thing was to have somebody try to stop me. I would recommend making yourself available to your friend at all times when they feel like harming and rather than stop them, offer to talk to them instead, as sometimes a distraction can help the moment pass.

Reassure her on how much she is loved and needed. That she does have a purpose and she is beautiful

Talk to your friend about resources she can seek out to get the help and support she needs from a professional

You can't make anyone do anything but you can offer support. If you're comfortable to, you can offer to be a source of support in those moments where she feels the urge. It can be useful for people who are self harming to know there is someone they can reach out to without judgement and can potentially provide a distraction to cope with the feelings causing the impulse. Perhaps you can engage in different activities to do with her, or things that she can do alone as an alternative. Of course you can't be her only support as that's a huge ask, so it could be useful to suggest that she seeks some further support from a professional who can help her through such things.

Support your friend through this rough time, make sure she knows you are there for her. Recommend that she see's someone, a counseller, or get her to talk to her family about her issues. Tell her that she always come on here and chat to one of the listeners if she needs to let it out.

My friend made me throw my blades away and I felt so much better. They were right on the phone comforting me through it all. I really think the best thing to do is encourage, guide, and comfort them.

Probably talk to her and express having challenges with what to do given that you know about her self harming. Explore options together and what you intend to do. Transparency with your concerns and helping them seek professional support can go a long way!