Wow! Scary. Couple observations from me.
I believe it should be a double fishermans (not a single fishermans or heavens forbid an overhand knot). According to grog knots app, it is easy to tie incorrectly and fail, so should be inspected. Double fisherman knot will lock tighter as more pressure is put on it.
As said, the knot should be offset from the carabiner connection.
It did not look like there was enough rope to have a proper double fishermans knot on the prusik cord.
This could have turned out different if a second figure eight was on his "tag end" and had that clipped to his carabiner as a backup.
Just curious, but I wonder how a ropeman would have handled (or not handled) this. I wonder would a second figure 8 not even help if the ropeman broke the main tether rope.
In any case, I wish him a speedy recovery and I hope it brings out a good conversation here that would help us all learn and practice better safety. Complacency kills.