I think we can all agree on the following (hopefully)
The terrain is suitable for both skiers. (Grey and red guy)
Both are skiing it well (though Grey guy looks a little more skilled)
Neither is skiing unreasonably given the conditions and the width, pitch and snow conditions on the trail. (I'm not speaking intent here.....I'm speaking their SKIING ability)
There is a question as to whether Grey guy reasonably changes turn shapes. So consider this;
Are there any skiers/boarders in front of him when he makes the change to the long radius turns.........NO
Does he appear to loose control......NO (perhaps in the split second when he realizes the collision is probable)
Does he use the entire width of the trail leaving other skiers no option but to stop skiing to avoid him.....NO
Is he entering another trail, turning to avoid an object, another skier, object or marked hazard....NO
Is he the skier ahead at the time of contact PERHAPS (but it doesn't matter)
An Expert (legal) would view the video and see the two skiers crashed into each other. While the code specifies the skier ahead has the right of way, in this case, it wouldn't be reasonable for the guy in red to ski into the intended line of the guy in Grey in such a way that there was no possible chance for the guy in grey to avoid the collision (which is what happens here)
Now........put aside everything I said above and do this. Rewind the video to the start. Watch the path of both skiers (and nothing else).
Grey guy is ahead of the guy in red from the moment they separate from each other where the trail widens. There is no point till the moment of collision where the guy in grey would reasonably see the guy in red WITHOUT turning his head to look UP the hill. I'll concede that it's reasonable for him to look accross with a radius change like that and he may or may not have done that, but again, it doesn't matter. The guy in red SHOULD have had him in his field of vision from the start of the trail where they separate paths as he is behind the skier in grey until the split second before the point of impact.
There is no question what so ever the guy in RED is at fault.