It might sound stupid but is she filling the gas tank with the motor
running ?
The oxygen sensor goes nuts when this happens thinking there is a
gas/air error and the check engine light comes on.

sonofadocker

I'm going to guess that an evaporative emissions code is being set..... I'm
going to guess that the code is for a minor leak.... do I seem to be doing a
lot of guessing?

Now..... minor leaks can be difficult to find, but not impossible... If your
dealer doesn't have a smoke machine..... there's been a waste of time going
on.... If your dealer has a smoke machine but doesn't have techs that can
follow some very simple directions (outlined in the PC/ED manual)... there's
been a waste of time.... If your dealer is good to this point, the tech
should use one of those powerful halogen spot lights - these can help to
make thin smoke appear....

At the same time, the tech should be checking OASIS to see if there are any
applicable TSBs... FWIW, I use OASIS a LOT. The TSBs and SSMs that might
come up can reduce diagnostic headaches amazingly.

I can understand your frustration at having such a simple concern dragged
out like this.... you might quiz a tech a little to see if he does know the
evap system.... First, the WDS or IDS is used to run the EVAP test (fuel
level must be between something like 15% and 80% of tank capacity - I can
never remember and always check the manual). This test will tell us if the
system is indeed leaking or not and can also tell us if it is a minor leak
or a gross leak. The Schraeder valve (usually located at the purge solenoid)
is plastic and has a left hand thread.... This is important to know since
the valve can be easily destroyed but still needs to be removed to smoke
test the evap system. When charging the system with smoke, I remove the gas
cap to permeate the system with smoke quickly - once smoke appears at the
filler neck, the cap is re-installed and the scan tool is used to duty cycle
the vent valve to the closed position. Pulsing the smoke machine makes
better smoke than just holding the button steady.

The other reply you got is cute.... it shows that the system is grossly
misunderstood....but it is cute...