Wait, a druid in whildshape does not revert to human form if their wildshape HP pool is depleted, they just die?
With NPC companions, yes. I haven't checked player character mechanics but I would expect to see the same apply to our NPC companions.
The NPC companion just outright died and the only way for me to have got them back was reloading a save or a scroll of revivify.
This was a story related NPC as well as they weren't some unimportant person I dragged from the tavern.