On May 18, 2013, about 1750 eastern daylight time, a Cessna 172N, N4702F, registered to and operated by a private individual, had a passenger contact the turning propeller while exiting the airplane after landing at Southerland Strip Airport (FD92), Geneva, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 Code of Federal Regulations (CFR) Part 91 personal flight from FD92. The airplane was not damaged and the private pilot was not injured. The passenger was seriously injured. The flight originated about 10 minutes earlier from FD92. According to the pilot, prior to any flights, he briefed each passenger as a group where to stand on the ground prior to boarding the airplane, and to wait until told what to do before boarding the airplane. He also briefed each passenger when they boarded the airplane, and before leaving the airplane. The instructions he gave during the exit briefing including advising each passenger to exit the airplane to the rear and also to tell the next waiting passenger to board the airplane from the right rear. Following each flight he kept the engine running. He flew 2 passengers uneventfully and then the third passenger boarded the airplane from the rear for a 10 minute flight. He briefed her upon entering the airplane, and then departed on a local flight. After landing he taxied the airplane to a spot on the field and with the engine running, he advised her to exit to the rear but to tell the waiting passenger that person would be unable to go because of weight issues. The passenger exited the airplane, closed the door, and instead of walking to the rear as instructed, rounded the right wing lift strut and walked forward. He yelled to her to stop but not before the propeller contacted her arm. He secured the engine and the injured passenger received medical attention then was transported to a hospital for treatment of her injury. The two passengers previously flown by the pilot reported to the Federal Aviation Administration (FAA) inspector-in-charge that the pilot briefed them that they were to enter and exit the aircraft at all times from the right rear and at no time should they be near or in front of the airplane while the engine is operating.