I had a friend who self harmed and when she was finally brave enough to tell her mother, her mom reacted similarly to this girl's mom and I can tell you, when she came to me after it, it was the most horrible thing to see her reaction. People who self harm want to be loved. Reacting angrily, yelling at them, punishing them for it, is basically encouragement. It makes the problem worse and can even make them cut deeper. If you care about them don't do that. Tell them you love them instead.