United Airlines does not stipulate limitations on the carriage of brachycephalic dogs such as Frenchies. His carriage was fully within the airline's terms.
I doubt whether the owners knew about the health risk to their puppy posed by flying. Modern airliners fly with a greater cabin pressure than those of earlier generations, so cabin altitude is likely to pose less of a risk than hitherto.
The agony and fear suffered by this poor little dog before he gave up his struggle for life must have been truly horrific. The family would have been traumatised and perhaps even the FA might have been shocked when she realised what her cruelty had caused, but neither suffered the fate of that poor defenceless little animal.
I hope that there's someone with a big enough heart in the USA who will offer a new French Bulldog puppy to that distraught young girl.