An uncontrolled UTH can reach temps of 130, and with something loose like aspen you can get burrowing as the snake moves around under the log. A snake directly on top of an unregulated UTH can definitely get some serious burns.

I'd get a thermostat as soon a possible. She may be choosing the log for it's safety feeling rather than temp. They don't always know to avoid really hot things(hence the reason hot rocks are such a bad idea)