Unfortunately, such low time pilots are unlikely to have had it happen before and it is unlikely to have been covered in PPL training.
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Surprises me as I've seen a few other accident reports starting with a door opening in flight. Friend with a fairly new FAA PPL had the pax door open on a C150 just after take off from Shoreham - passenger held the door as near to closed as possible, they did a circuit, landed, closed it, took off again. I think they both had a quick go at closing it in the air but he said it was no real drama. Whenever I flew with him he always insisted on a good shove on the doors as part of hatches and harnesses secure before starting.