If the CEL was set from the gas cap being left off, it should reset itself after a couple of drive cycles. If it's been on longer than that, she needs to have codes read to find out what's setting the light (it probably won't be the gas cap).

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]