I can tell you it's not serious. It's almost likely a problem with the Evaporative Emissions system. Half a tank or more of fuel, all that gas sloshing around in there creates a lot of fumes. The job of the Evap system is to vent excess fumes into the engine at certain times , which helps on fuel economy and emissions. There is a long vacuum line that goes from a vent in the gas tank to the purge canister up front. The canister is what collects the excess fumes. There is another vac line going from the canister to the engine. In between are a purge solenoid (which opens to let fumes through) and a purge control valve (which controls just how much fumes get through to engine). Now that I have explained that, it's important to know that many things can trigger a fault in the Evap system and set off the CE light. It's either the canister, the purge valve or solenoid. Or a disconnected vac line to the canister (which I doubt in this case). A loose gas cap can trigger it also if the tank vents too much through the cap and not to the evap canister.
It's a common issue on 94-98 Mustangs. The evap system, usually one of the things I mentioned. Have (him) get it scanned at a parts store like Autozone or Advanved Auto Parts. They can usually come outside and pull the codes for free. The fault code will probably be something in the P1443 area. If he lives in California tho, he'll NEED to have the fault fixed. Their emissions laws are hell strict.
Thanks very much. I suspected along those lines but am not familiar with the machine. That it is common is what I was looking for. I would think vac. line or solenoid electrical connection is most likely, followed by bad solenoid.
The purge solenoid is quite easy to test if you remove it from the vehicle, a 12v source should open the valve & you should be able to blow through the ports (not by mouth!). Clean the solenoid by spraying carb cleaner & shaking the carbon debris which should loosen & run out , this can impede operation of the valve. Also blow out the purge lines (disconnect first) also check for cracks in the lines & evaporative cannister. There could also be a Purge flow sensor which can become faulty. You can also check the signal to the purge solenoid with a test lamp, If you have an assistant, set the engine at about 1500 RPM when warm, back probe the signal wires, the purge signal voltage should come on intermittently when the engine is at operating temp.