THE PLT STATED THAT AFTER LNDG AT NIGHT, HE STOPPED THE ACFT SHORT OF THE PARKING SPOT, INTENDING TO SHUT THE ENG DOWN TO REMOVE A VEHICLE FROM THE PARKING AREA. HE RPRTD THAT BEFORE HE COULD SHUT DOWN THE ENG, THE PASSENGER DEPLANED & WALKED INTO THE PROP. HE LEARNED SUBSEQUENT TO THE FLIGHT THAT THE PASSENGER HAD CONSUMED FIVE FIORNIAL #3 CAPSULES CONTAINING A TOTAL OF 150 MG OF CODEINE AND 250MG OF BUTALBITAL BEFORE THEIR 1900 CDT DEPARTURE.