One recourse he could take that would end it all is to find out who holds the Mercers home owners insurance and write them a letter complete with photographs of the trampoline without a safety net fence around it set that close to the house and hard rocks. They would force the Mercers to either remove or modify that situation or they would stop insuring them. Then the mortgage holder would come down on them like a ton of bricks. All he has to do is point out the obvious safety issues to the insurer. Insurers hate trampolines.