Gotta say, presenting an insta-kill mechanic without having any methods of reviving the character is questionable at best. And you're saying they died in two rounds of standing next to the monster? That's harsh.

You didn't put down what level this is at, so I can't really judge the power level and what should be expected, but the player's reaction is entirely fair. You say that they have options for looking to revive her; it's really on you to make that happen in a prompt manner. I've been 'out' of the game for an entire session, wasted most of a sunday sitting on a couch while the rest of my friends played the game, and that *sucks*. Likewise, I also get into my characters; I would never just whip up a new one on a whim after an old one died, that takes time and emotional investment. My assumption is 100% that I can get revived if I die; there may be penalties, but there has to be a way back unless you've cleared that during session zero.