Man killed in skydiving accident wasn't planning to jump

Yes and No. The accident was caused directly because he was wearing skydiving gear. The premature deployment resulting in his ejection would not have taken place had he not been wearing a jump rig. Yes, technically he was probably not wearing "skydiving" gear, but an emergency parachute.
Also, emergency rigs are not required for passengers in skydiving aircraft unless the aircraft is doing "aerobatics". Observers on skydiving runs wear parachutes because pilots crank the planes down for the next load, typically exceeding the bank and pitch angles for non-aerobatic flight as defined by the FAA.
The PIC is totally to blame for not making him wear a seat belt, and letting a non-trained jumper to sit near an open door with a rig.. He was damn lucky the parachute did not snag on the tail or stabilizer. Premature deployments often result in loss of control of the aircraft.