I doubt that a temp is spec using an IR gun anywhere on that generator. So no real answer can be given.
I assume it was under load and fully warmed-up before IR temp reading? Is the mechanic trying to say it has a bad T-Stat, low on coolant (air pocket), bad water pump or bad design? If he's using and IR gun, he should be able to tell you why it's running cool since he "knows" it's running cool.
Seems a water cooled generator should have a temp gauge.