Realistically the rider on the animal is riding at their own risk. Regardless of the level of experience you are always riding at your own risk. Now having said that, legally the owner of the horses and the farm could be sued(sp??) because the incident happened on their property.
As a horse owner you are responsible for taking on horse insurance to cover for accidental death and injury of a guest when they are on your property. I know it's something I've always carried while owning horses. The farm insurance(or house insurance) should also have coverage for accidental injuries of guests on the property.