Nobody likes being "told on" to an adult, but I'm sure that even if your friend gets mad, that eventually they'd appreciate the help. Going through self injury by yourself and trying to cope is HARD.

As for what YOU can do, how much do you know about self injury? There are a lot of good books out there that may help you understand. And if you so choose, you could also give your friend the book after you finished reading so they know they're not alone in dealing with this.