Well she said she wouldn't, so that's at least something. Is this the first time she's cut before? If it is, I think there's a good chance she'll stop. If not, I've learned from personally dealing with people who've made it a habit to never take their word for it and ask them constantly. Either way, just keep checking on her, let her know you're there to support her always, and do your best to help her feel better about herself.