My mother has also had problems using the Netjets code at ZRH. The manager told her that only an Avis employee code could give her such a good deal!
When I book a car for her nowadays, I find B149870 gets the second-best rate and she's never been hassled using it. Still, it's a shame to have that great code go unused!
Would anyone know how one could "prove" the code isn't an employee code. Is there some sort of Netjets literature she could bring to airport with her?