Like T&F and Dante said, as long as it's just cutting the surface of the skin, it really isn't doing her any harm (as long as she's using clean knives).
Just make sure she doesn't do it when completely pissed, like my housemate
.
I'm sure there's oogles of sites out there more qualified to answer your question than us anyway pig. If you really want to help, maybe get some information from one of them, print it off and give it to her with an "just thought you might want to read this" comment.