I think the disagreement is about who is ahead and who is behind, and stems from the limited camera view.
We see grey making turns with his overall direction of travel down the slope. Grey's overall direction of travel changes from towards the camera to follow the fall line, coinciding with a medium turn at the beginning of the video. Then we see Grey make some short turns, his instantaneous direction changing slightly to the left and right, but his overall direction being down the slope while he is making rather shorter turns. Then we loose sight of him, but pick him up again later when he is making a larger turn with his overall direction still down the slope, as observed if you can see the clues in his stance (e.g. counter, balance, direction of glance, etc) and relate it back to his previous skiing.
Red likewise has an overall direction of down the slope, but is consistently skiing on the left side of the trail and doesn't change his turn radius much, nor ski very far out of the fall line with his turns.
If you use the instantaneous direction of travel, for the entire video up until a brief moment before the collision, grey is ahead of red. That brief moment, the time it takes to swerve left when swerving back and forth using a run that wide while skiing down it and keeping your upper body aligned with your general direction of travel, and your vision focused on what is ahead of you in your general direction of travel, is seen by a few here as long enough to count, and by most here as not long enough to count. Some of us are lucky to be endowed with quick reflexes and good skills, but most skiers would not IMHO be able to avoid that collision in Grey's place, given the time between Red entering Grey's reasonably expected field of view and the collision, given that Grey is skiing in a the general direction down that slope. To expect someone to look all the way behind them every time they make a turn is too much, almost nobody skis like that.
If you use the general direction of travel to define ahead, then red is not ahead of grey, until after the impact.
Either way, Red could see Grey in plenty of time to take evasive action and should have adjusted his course, either not pass Grey in the direction of travel due to lack of skill to avoid Grey if Grey turned left, or pass Grey on the right as Grey turns Left.