So, when you put the plane on autopilot, you just stop paying attention then? Open the books and let the plane fly itself? Seems to me ground instruction ought to be done in a distraction free environment, whether that is in an FBO, house, boat OR like
@RussR said, you are both on a commercial flight as pax. It definitely doesn't have to be inside an FBO in my opinion.
Ground instruction is also usually prep work, or general concepts. Maybe you go through some stuff inside the plane, but the engine does not need to be turning costing you hobbs time or operating time to do ground work.
FAR 61.1:
Flight training means that training, other than
ground training, received from an
authorized instructor in flight in an
aircraft. (Technically
@RussR's example would be this as it doesn't say you have to be the one flying the aircraft lol)
Ground training means that training, other than
flight training, received from an
authorized instructor.