The airport authorities should counter that the family knew or should have known that she would never make it to her destination sober and therefore should have accompanied her, or sent someone in their place.
I think you have established one element of “but for” causation, but it’s unlikely a court will go that far back to trace the “proximate” causation necessary to find negligence on either side of the “vs.”.
I think the act of detention by the airport authorities is going to be deemed as a superseding cause, especially since the handcuffs were the instrumentality of death. It's from that point on they are likely to look at fault.