Having the wishing stone go to a whole new group of people is easy enough. The stone could get stolen by a bird, get lost in a hot-air balloon, train, taxi cab, or other means of transportation. Or Jon and Karyn could get so scared by the stone and how dangerous careless wishes could be that they wish Jon's grandfather had sent it to someone else.
There are lots of different ways to dump this particular problem on other people. :)
