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.