A new gas cap is cheap and couldn't hurt. However, if there is no gas smell in the car while idling for 10 minutes in a parking structure surrounded by parked cars, and the odor is only there when idling in traffic jams near other idling cars, then he probably isn't smelling gasoline from his car, IMHO.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E