Kidneys directly affect heat sensativity. The occasional kidney stone indicates problems with the mineral balance in his entire body. That's a problem that needs to be addressed otherwise he'll start having many other problems before he hits 50.
Anything "Diet" has aspartame in it. Nasty stuff. A strong neurological POISON.
If he's in pretty good shape now he can do the "Baiscs" program. Get a hold of Barefoot and do his kidney regime. The heat sensativity issues are a warning shot. Take heed. Many other problems are on their way if he does nothing.
If he choses to do nothing make sure he's well insured. Don't want to scare you just giving you ample warning.