When my dad found out about me cutting, he made me promise that I'd never do it again, which was sort of unreasonable, so that did nothing but turn me into a liar... Then when he found out I did it again, he asked about it every day, which just made me think about it, which made me want to do it again, and it got worse. I hope you can give her space and make sure she knows she can talk to you about it with you when she's comfortable, just don't be pushy about it. I'm sorry this happened, it's hard...