A descending turn will reduce both radius and G force, if height is available. Start by climbing to reduce speed, which will build up in the descent. As you are turning down valley you can afford to go near to the ground. And use any extra width from side valleys if available.
There is the danger in any canyon turn that there is a downdraft on the side you are turning to.
It has to be more than just descending. You need to accelerate in the descent. It is quite doable if you have height, and if you unload the wings enough, you can do a really fast tight turn, safely.
You could also do a Chandelle.
I did a huge number of those for the FAA CPL. It needs plenty of speed before you pull up. Yes, sure, if you have speed then you have even more options. There are all kinds of “aerobatic” turns, for those who know what they are doing. But all need speed. Abd most sensible people will slow down if there is terrain left and right – so they can do a tight turn.
Actually the entry speed for a Chandelle isn’t excessive, eg 105 kts for a C172. The alternative is power and the guy was flying a 182RG which has plenty of power, so probably doable. That said, you need to be trained for it.
One more thing. Why did he even turn? This was not an escape maneuver, the valley goes on in almost a straight line. Therefore, this seems to have been a misjudged intentional turn. Looking at the profile of his previous flight, that seems plausible. Doing this in mountain valleys at low altitude of course is risky business.
Therefore, this seems to have been a misjudged intentional turn.
Could be anything. Stall/Spin? Structural failure? Pilot incapacitated?
