I've noticed here on the forum rocketeers get quite testy when they see videos of people doing stupid things with rockets, and the NAR has strict safety rules and guide lines for the hobby all for good reason. As with this hobby and any other high velocity hobby safety MUST be top priority. These things are serious business, HIGH powered, and demand respect. I've learned this first hand already, just the other nite I was reprogramming the transmitter on mine, in the living room, I had the area clear, and the heli taped to the table with large strips of Gorilla Tape, good thing to, when I plugged in it spooled up full throttle, even flipping the throttle cut switch did nothing, I had to shut down the transmitter to kill power. If any one would have been close to it, or had it not been taped down with tape that actually peels the wood off the table when removing, things could have been bad.
I treat this machine with the utmost respect. The chopper he was flying looks like the T-Rex 600 or maybe even larger, way more powerful than the one I have. This incident is the exact reason I haven't tried to fly mine yet. There's too much I don't know yet, and when I do start flying it's going to be low and slow.
Not knowing the details here it's very hard to say what could have happened. Mechanical failure, weather, but he sounds like an experienced flyer, and builder, I would hope it wasn't caused by over confidence to the point of losing respect. This is usually when accidents happen, getting over confident in any thing and losing respect for what ever you are doing, and this applies to work place, driving, hobbies, sports and any thing that is potentially dangerous.