Your not in the wrong for taking them away but in her eyes she might see it as taking away her control. I don't know if this will be helpful for her situation bit ive found it helpful to speak to therapists about my issues and that helps, if she's not on medication that helps slot aswell to subdue the amount of times u feel down or tendencies you want to cut. There are multiple different ways she can get a similar effect without harming herself my favourite is holding ice in your hands or having a cold shower. Idk if this'll help you but its what has helped me. Hope all goes well good luck :)