Very unfortunate incident that should have been completely unavoidable. Fortunately no serious injuries and I am sure that the pilot has learned a great deal from the experience.
My observations are that simple basic airmanship could have prevented the factors lining up, long before the unfortunate development of a Vortex Ring state.
For what its worth, I have the following questions:-
1) How extensive was the legally required pre flight briefing? Delivering it to passengers when rotors running, is too late and never satisfactory.
2) Why was the door's poor latching not noticed during pre take off checks? It was a front door, so the handle would have been visible had it been checked or the passenger properly briefed.
3) Why were dual controls installed when the passenger was not a qualified instructor? The only reason is the provide the opportunity "to let a passenger have a go." Foolish!
4) Flights involving off airfield landings and "pleasure flying" are not the norm for PPL's and seeking advice from a more experienced pilot/instructor is always worthwhile.
The comments about training pilots how to deal with an open door are valid, we do not cover such incidents as a matter of course BUT don't tell me the pilot was not training to ensure that all doors were securely shut as part of the pre take off checks.
This is precisely the sort of accident that comes about with low time private pilots taking on challenges without adequate preparation. It is what terrifies me with this insane Wingly idea as referred to in the posts above. Commercial Pilots and Operators spend a fortune getting qualified to take on such flights and are subject to excessive, draconian oversight by the authorities and yet PPL's have suddenly been given a method to fly in this manner with absolutely no support, control or oversight.