I've definitely lost friends over the issue of cutting. It sucks, but in the end, I learned that if someone would terminate our friendship because of that, their friendship was not worth it to begin with.

Most people don't know how to react to self harm, and most don't understand it enough to even try and react to it properly. Maybe it would be beneficial to sit down with your friend and explain what you can about it, and try to help him understand. If he is a good friend, he will listen to you.

Good luck with this!