It's a combination of both; driver and vehicle.
4WDs on sand particularly DRY sand up the East coast are whole new animal and these toys just can't hack it. As I said in the title, the drivers ain't none too bright normally and the general impression this side of the pond is that if you can't learn to drive properly, then option 2 is to buy one of these impractical around town cars and work on the principle that everybody else will then get out of your way.
This usually works well until either another of similar talents driving one comes the other way, there's a tree or other obstacle that can't get out the way or they decide to try to emulate the ads on TV (as displayed here); then common sense clicks in, something that anybody buying a mythical truck to drive around town obviously lacks.
The area where she went down I know well; it was my old stomping ground as a teenager and the tracks she followed were made by a pro fisherman in an old Land Rover with balloon type sand tyres with around 10 psi in them that roll across the top of the sand. The toy by comparison has thin tyres running around 35/40 psi which cut nicely through the sand.
Doubtless, the toy has constant 4WD with no upper and lower range, so doesn't have the ability to crawl across the really soft areas, so when it begins to sink, they give it a bootful of power which drives the skinny wheels further into the sand and the fact she was down below the high tide mark tells me that is exactly what she did, but in a proper 4WD you can get out, but these wanky round towners is a whole new ball game.
As I sometimes get called in on retrieval of 4 WDs stranded on Fraser Island I see it all too often.
Don't know how they'll stand in regards to Insurance; could be a very expensive lesson in common sense and why not to buy soccer mums anything but a real car.
Incidentally, the sand "that soft" is below the high tide mark and when we fly to the island, that is where they land the aeroplanes on. Leading to it is where the really soft stuff is as it's dry and not compacted and is easily driven on with vehicles with real 4WD transmissions and tyres. She obviously rolled through going downhill and started to bog just before the water line which was when the panic set in.
Alan S