Jennifer Riordan who was killed when she was accidentally blown out of a Southwest Airlines plane, actually had her seatbelt on at the time of the incident. She died due to the incredible force of impact and trauma that acted on her body, according to authorities.
This Myth Busters video demonstrates what happens to passengers during an accident like the one Jennifer Riordan suffered.
Here part of the plan.
Here you can see the dummy pressurized out of the plane.
Here is a still image of the explosion.
Here is the remnant of the plane after the second trial test.
Here is the entire episode by Discovery.