I would be looking for something in the gas line as it seems there doesn`t appear to be enough pressure to keep the burner alight when the door gets shut.
Get him to check working and standing pressure first as well as all the air vents into the oven burner chamber.After that it may be a matter of disconnecting a blowing through all the gas pipes going to the oven and checking the thermostat.
Gasman
September 2008