That sucks, I've seen a few worse then this with mechanics and ladies. had one that charged a older lady $50 for a seat belt adjustment. FYI abs codes are a 2 trip issue, might have set a soft code and not turned the light on, as for the diagnostics, that is a bit excessive and i'm guessing they bought a whole case of dielectric grease for $32.
FYI here's what gm says that code is :Reverse Gear Signal Circuit Not Plausible. If you would like more info PM me.