Forgive my ignorance, but was that a legit guard pass the other guy did?
It looked legit to me--only executed with excessive force and a lack of awareness of the kid's precarious position. It wasn't a pass, but white was trying to flip black and probably try to spin to his back. Likely he was expecting white to move his head to a safe position, which is at least partly the defender's responsibility, just as landing safely when thrown is to a great extent the responsibility of the player being thrown. That said, white did come down hard...I could imagine him getting disqualified for an illegal slam, had the circumstances not already rendered the match itself irrelevant.
I have got myself thrown on my head (thankfully not in such a bad position) as a result of trying to maintain position on my feet when I should have given up, moved my head out of the way, and simply tried to land on my back. Similarly, I have put myself in a very similar position to this (minus the dangerous downward momentum) when being stacked and trying to maintain guard--there is a balance between thinking one can maintain the position, and knowing when to give up for safety's sake. It was a freak accident, but both parties could have been more conscious and prevented it, not to mention could have been in different divisions...is it common for mixing like this between adults and teens?