Most of us will have experienced a problem with doors coming open in flight and managed to sort it out without too much drama (whether through luck or skill).
Unfortunately, such low time pilots are unlikely to have had it happen before and it is unlikely to have been covered in PPL training.
The accident was entirely preventable - through training of the pax or the person closing the doors as Rotorspeed and RINKER say - (every passenger in a military aircraft has to have been given an aircraft abandonment brief including opening and closing of the doors).
The fact that the pilot was overwhelmed by the situation was just another item in the list of human factors that so often lead to private pilots reaching their capacity limits with no-one to save them from basic airmanship failures.
Fortunately just minor injuries and a red face this time.