I believe this counts as self-injury but I would really like some advice.

My friend punches herself until her skin turns purple, and she has done this so frequently that she has lost feeling in certain parts in her arm. This is a little bit different from other forms of self-injury, but I know that it is still self-injury. I have plenty of examples with how to deal with someone who cuts or burns as a means of self injury... But how do I go about helping her in any way?