It seems like a point of perspective here, the friend who told her clearly doesnt know what it is like to go through the self injury struggle, but you have to try and see that they probably did it to help you out and because they were scared of you going too far maybe?

I can totally see why you are feeling betrayed though!

As for your mum, why dont you write her a letter and explain what is going on properly with the SI, put it in a way that she might understand a bit better?