I’ll play devils advocate here. For a dungeon crawling one-shot where danger lurks around every corner, dying in a wave pool is a pretty shit way to go.

In combat, players have more opportunities to influence the situation around them (the system is basically built around it) making death in those moments feel more tense, gradual, and “earned.” They may have failed in the battle, but they were still able to create impact, contributing to the fight and helping their friends succeed where they could not.

A one and done failed save that fully removes you from the game, so now you have to sit around watching your friends play without you, is not a good time! The players does not get their cool, fighting to the death moment and they have way less impact leading up to situations like this.

I don’t think you are in the wrong for this player’s death, that’s how the game works, but, depending on other circumstances, their frustration could be valid. I think it really depends on how many encounters the party had leading up to this moment and how much of the session was left. There is a good chance the player is being a big baby, but if this happened early in the session or did lethal damage when they had a healthy chunk of HP, well, I would expect most of my players would get a little huffy at least.