I've seen a lot of very angry posts about the assistant ref suggesting the red, but I agree with this 100%
Some have said "he didn't clearly see it, he can't give the red", and on the face of it, this seems compelling.
However, can you imagine the absolute fury if one of ours took a punch, or an elbow, or a kick, with a player's body slightly obscuring it, and the ref didn't give it because while he saw the punch/elbow/kick, he didn't 100% see the contact? If there's a swing, and a player hits the deck, you may well need freeze-frames to actually be 100% on the contact.
It was a little naive to let his arm swing like that, but I give 100% of the blame to the player who hit the deck.