It's true that professional horseback riders generally have more back problems than people who don't ride horses. Research has shown that horse riding can affect the way the spine ages and changes as a result of physical strain on the spine from riding. This is especially true for riders over age 35. These older riders tend to have more low back and neck problems than younger riders. 

Horseback riding can be a dangerous sport. However, its dangers to the spine seem to be related to the total time and years spent riding. In other words, the longer your daughter rides, the greater her risk for back and neck problems. In the meantime, a trainer or physical therapist can suggest ways to minimize the stress that riding places on your daughter's back.