If he filled the tank and didn't screw the cap on correctly (three turns until you here the rachety clicking sound), that will set the CE light off.

Recheck the gas cap first before you panic...

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car