Pre book the parking & make sure both people have a key for the car & the booking reference.
When you drive up to the barrier the number plate will be recognised, a ticket will be issued & the barrier will lift.
Leave the ticket in the car & let person b know roughly where the car is parked.
Person b arrives, picks up the car & provided the number plate is recognised on the way out there should be no problems, if there are problems you've got the ticket & the booking reference which should be recognised as being pre paid.