The inverter is not I use when the genny is supplying AC power. Well it could be, but doesn’t need to or shouldn’t really, IMO.
GFIs can be defective and wear out or they can pop due to a ground fault.
Age, amount of use, etc we don’t know that from your description.
Regarding $2500, if he’s been at this 40 years and gets $2500 for repairs like that he should be retired on a beach, that he purchased! Lol.
The most expensive 2000W inverter you can buy is around $600 and it’s about as difficult as a car audio amp to install except maybe easier.
$2500 is about double what I’d expect at least.
