Is it possible that he is not getting as much exercise as he's used to? That's what it sounds like to me. If he's used to having a yard and paths to run on and expend his energy and isn't getting the opportunity to get rid of that energy now, then it could lead to destructive behavior like what you've just mentioned. Since she has seen a vet to rule out a medical problem, tell her to try increasing his exercise, and mental stimulation.

Best of luck to her.