I see more hock issues in jumpers first before back or front legs, but every horse is different. It is determined by a lot things; such as age, conformation, maintenance, equipment, rider and luck. That said, unless the jumper schooling lessons are incredibly long and taxing, jumping 2x a week at 3' or less should not be too wearing on a healthy fit 9yo horse.
As for the 'lease', if there is no money exchanged and they are not paying for upkeep on your horse, then I see this more as you having someone exercising your horse. Your horse, your rules. If you do want someone to jump your horse, then you let them know that this is a flat work arrangement only.