I get the same mouth ulcers, and they are horrible. The only treatment I have been offered was something they called "magic mouthwash". It's basically a mouthwash made with topical anesthetic for oral pain. Personally, I hated it. I don't like my mouth being numb, and that's all it does. It doesn't work for very long, either. My preferred methods of treatment are diluted listerine or salt water rise. For me they too only offer temporary relief, but it's better than nothing, and won't make you feel like your dentist just shot you full of Novocaine. I was never told how to prevent them, or if they even know how to.
I wish I could offer more advice. I know how miserable she must be right now.