This sounds like a slow leak in the CO2. As the CO2 turns from a liquid to a gas it gives off heat and can get quite cold. I had this happen when one of my kegs sprung a slight leak through the top. The CO2 tank developed a coating of frost on it. I don't know why that would affect the regulator however.
The other option is that his Keezer is below freezing. Frozen beer would be hard pressed to carbonate.
I've also seen frost on the walls of my keezer when it goes through a cooling cycle. But with the external thermostat the air temp stays at the set point.
I'd advise them to check for leaks and try turning the temp up.