I've not heard of it being used to treat mental illness, but it's been years since I have had anything to do with medications. If I was to hazard a guess I'd say perhaps it is to counteract side effects of her other medication, though I could be totally wrong. Does your daughter have any known heart issues?

When I was a young kid I was put on Inderal for heart issues, and it gave me wicked night terrors. If your daughter has been put on it very recently, depending on how old she is that may be something you'll run into with her.