Self harm is a coping mechanism. Everyone needs coping mechanisms when they are dealing with something tough, but most people choose something less destructive, like playing videogames, or talking to friends, or cooking delicious cakes (I'm hungry, shh).

People do it for various reasons - I've hurt myself in the past, but it was more of a compulsive thing, acting out of sheer frustration at the world and my inability to deal with it like "normal" people do.

Anyway, the important thing here is that she's upset, and sad, and hurting inside. Help her deal with those feelings in a healthy way, if you can. There are many resources on the internet for this, or you could ask for more advice from a helpline, etc. Talk to her, let her know that you care, and that you want to understand how she is feeling. Perhaps ask her to contact you (or someone else) if she feels like hurting herself, and talk through the problems (but be gentle...psychoanalysing someone when they're deeply upset is not the best idea. Leave the advice and opinions for a time when she's feeling calmer, and stick to listening, understanding, and just letting her know you're there for her when she's feeling that bad).

Remember though - if she does hurt herself, IT'S NOT YOUR FAULT, YOU ARE NOT RESPONSIBLE FOR HER ACTIONS. Take care of yourself, too.

Sorry if this is a bit of a disjointed reply, I'm really really tired. I hope what I said made some sort of sense. You sound like a great friend to care for her and want to understand better :)