If you are certain that he didn't put gasoline by mistake, tell him to check the gas cap. Make sure that they put the cap correctly. Once everyhting in order disconnect negative from battery for a few minutes and reconnect. Check the instrument cluster for engine light. If off and reappears then you definetly have some problem and it will have to be scanned. If it doesn't light up your ok and it was probably the cap that wasn't secured correctly.