Jan. 13/14-The Air Georgian Beechcraft 1900D (C-GORC) was at the gate with no engines running and one pilot aboard completing the flight’s paper work. An aircraft maintenance vehicle from another operator was driving by C-GORC when they noticed the right propeller on fire. The driver got out and extinguished the fire and then reported the event to the pilot on board the aircraft. The pilot turned off all electrical power on the aircraft and exited the aircraft to evaluate the propeller damage. One blade was substantially fire damaged and the remaining blades showed deformation from the heat. It was determined that the first officer had inadvertently left the propeller de-icing system on prior to leaving the aircraft to perform a pre-flight walk-around. In addition, C-GORC has kit 29-3006-1 installed. The kit includes an engine oil pressure switch which activates when oil pressure is low, such as engine not running, and prevents the de-icing system from operating even if selected on. Maintenance found that the pressure switch was faulty and allowed the de-icing system to operate. The pressure switch and the right hand propeller were replaced and the system was tested with no further faults found. The aircraft was returned to service.