are the blend doors actuated by an electric motor? or are they vaccum operated?
Also, some heater valves are vaccum actuated as well....
I would tell him to get the engine to operating temp, and then feel the inlet and outlet hoses of the heater core.
Then have him drive to where there is no heat. Open the hood and then check the inlet and outlet hoses again. When the heater stops working, where is the engine temp reading at? is it as easy as a thermostat swap?