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I believe he’s saying that his friend has a GPS for small private plane use that provides altitude data, and chooses from an internal barometric pressure gauge and altitude readings from satellite data, and that the device uses the pressure as the default preference. This would, of course, be rendered fatally stupid in a pressurized cabin.
It was a little hand held GPS unit for use by private pilots in unpressurized cabins. It incorporated a barometric altimeter, and the capability for the pilot to input a local reference altitude to account for variations in surface air pressure. Normally pilots can adjust their cabin altimeters to indicated the known altitude of the runway before take off to account for changes in local barometric pressure. Airliners are pressurized to about 8000 ft., which means that when you are flying at 30,000 feet a barometric altimeter inside the cabin with you will indicate 8000 ft.