Except that in this case things deteriorated very quickly, by all accounts. So the pilot may have been surprised rather than overconfident.
It would really be very frightening to be flying up the narrow canyon and suddenly be entirely in clouds, with downdrafts. Yet apparently that was the situation.
So I'm at a loss for how to prevent such accidents. The risk seems built in given the terrain and the local weather.
As I said earlier - you always retain an escape route and if that means a bugged heading on the HSI which you know you can turn onto and fly out , even IIMC, then even if the weather closes in around you , you have thought about your actions before the situation occurs.
Weather decisions are easy - can you see where you need to get to? No? Then can you get there by going lower or higher to see your destination? No? Can you approach your destination from a different direction? No? Can you wait for a few minutes for the weather to clear? No? Then that is the weather decision made.