Aspen does a great job with humidity. ive never had a bad shed using it. only when i used sand (i know bad JeffJ, BAD)
you can get a lamp dimmer that will work on a heat pad just fine at lowes, Home Depot or any other place like that. a dimmer is like what it says. it dimms a lamp, (restricts current to the device) there for making the heat pad not run as hot.
a thermostat its a device with a temperature probe that monitors the temperature and either dimms or turns the heat source on and off as needed to keep the target temperature. the lamp dimmers are about $15 and 300W it will work just fine for nay heat pad and or lamp combo. just be sure nothing over 300W or it becomes a fire hazard as your overloading the potentiometer ion the dimmer.
as for the heat under the rock. its not uncommon for it to be a lil hotter in there as the cave will gather heat. if your friend stops using the cave its because its too hot. tone it down a few more degrees. and anything over 100*F can be potentially lethal or cause irreversible brain damage to the snake.