Can I be sued by someone who was injured while riding my horse if there is no negligence on my part as the owner of the horse?

I boarded my horse at a nearby stable and was attempting to sell her. One couple rode her without incident, and wanted to see her a second time. They brought a friend with more experience to ride my horse. When the friend mounted, my horse spooked with no warning and the girl fell. She injured her elbow, and is now threatening to sue for compensation of medical bills, time missed from work, and "pain and suffering." There was no negligence that lead to the horse becoming frightened, nor had the horse ever shown this behavior before when being mounted by a rider. The first time the boyfriend of the couple rode her, there were no issues at all. The injured rider herself stated after the fall that "that's the risk you take when you work with horses." If owner negligence did not cause the accident, does she still have any sort of case against me?