While deplaning; a passenger turned right into a B737 galley and; with the Flight Attendant calling to her; out into a provisioning truck before realizing the error; turning around and exiting the main cabin door.

Narrative

During the deplaning; I stepped in the forward bulkhead (aircraft right) to step out of the way of the provisioner and agent as they conducted their jobs of provisioning and paperwork exchange with the pilots. It was raining fairly hard as well; and water was blowing in the forward galley door. I was saying goodbye to the passengers. Toward the end of deplaning; a woman who looked to be in her twenties; turned right to exit the aircraft instead of following those people directly in front of her who had turned left and exited properly. I immediately caught her turning and began to say repeatedly; 'NO; NO; NO; NO!!! Ma'am please turn around and go the other way'. The passenger continued on and stepped out onto the provisioning truck; finally stopping; turning around; getting back on the aircraft; and exiting aircraft left. The passenger never said anything to me or any (company) personnel. I should have been in the galley. It is very difficult when at times there are 3 crew members; provisioning and agents all jockeying for the same galley space. Perhaps there is some sort of temporary boundary that could be placed between the galley area and aisle to help passengers avoid mistakes. Although; I'm sure this would be considered a safety hazard.