From what I've read about biofeedback, it can be used for many conditions and usually has high success rates. If her doctor recommended it, I think it would be good to try. I hope you can get your HMO to pay for it.
On another note, is there any chance she has
Rheumatoid Arthritis? Probably not, as that's something a pediatric rheumatologist would have tested for. I only ask because I had a friend when I was about her age who had similar symptoms, but that was because she had rheumatoid arthritis.