It does sound like a carb problem. Typically propane generators do not have the normal sticky gue in the carb, but in your case - probably lack of use is the problem?
My guess is to have the new owner run the generator with a electric heater as a load (less likely to get messed up by a wrong voltage or frequency, and very cheap to replace if it does). The 1,500 watt load will put some load on the generator without overloading it. The hour or so of run time should put some fuel through the generator carb, and clean it out.
If it is still messed up, then a pro needs to look at rebuild or replace the carb, if that is what they feel is wrong with it. Anyway it will work under load, so it is not your problem. .
Fred.
