During the course of the flight the pilot learned that the passenger in the right hand front seat held an American pilot licence and was experienced on the type. He allowed the passenger to manipulate the controls until the aircraft was on final approach and allowed him to keep his hands lightly on the controls during the flare and touchdown. During the landing roll the passenger, unnoticed by the pilot, inadvertently selected the landing gear up. The aircraft slid to a halt with the gear partially retracted.
