You have no further responsibility. Unless it's a personal morals issue.
Allen I've been in the boat business a Lot of years, and in the beginning when one of my customers had an issue with one of my boats I felt responsible in some respect, even though I took them out on the lake and demonstrated the boat, but things have changed, I got a different out-look now, once beginning, I sold the boat, took the guy out for a test run, he ran it through the limits, paid me and took it home. Two days later he called me up from different lake, he was dead in the water, motor shaking, popping thru the carb, then died, swearing he had Not had it in the water since the test run we did ... well, quess what, another friend of mine told me that he and the new owner ran the boat for 4 hours on the lake, full throttle, jumping waves, racing other boats, for a whole tank of gas the evening I sold him the boat, and too, that he demonstrated the motor to several friends on a hose in his driveway the evening befor he had an issue, taching it up to 6000 rpm. that being responsible for bending push rods, breaking two rocker arms, and a blown head gasket.
You should Not feel bad about representing your boat, it ran perfect, a beautiful boat, what ever happened to that boat when it left your care is Not known. You did your part, and you gave the responsibility to someone else, it's their proble, and they also probably created the problem.