Gotcha. In such a case, I would think PC death is on the table (no pun intended). That's a risk just about any time there's combat, especially against dangerous foes.
If not the result of combat, I was imagining some adventure modules from the old days that may have started off like this. In a situation like that... the game begins with the PCs prisoners of the mind flayer and there's a 5% chance he just eats one of their brains... there's no agency happening. It's just GM fiat to open the game that way, and GM fiat to decide there's a chance it eats a PC, even if he makes it a slim chance. There's nothing the player can do about it, nor did they do anything to wind up here.
But in a more combat fallout type of situation, the dice didn't go the players' way... so that's the risk. Now, obviously, the consequence of failed combat is up to the GM. They could have the mind flayer eat the character's brain, or dominate them to make them a thrall leaving the possibility they can be rescued and restored, or they can just leave them for the creatures of the underdark to feed upon, not wanting to consume the gray matter of such weak beings... leaving them alive...
So the GM has a wide range of options of what to do here. How exactly does he make the decision? I mean, any of the above options seems to make sense. They all seem equally plausible at face value. A mind flayer is also an inscrutable type of creature... we can't truly understand its wants or motives. Perhaps something was established during play that makes deciding how to proceed more clear.
But absent that... what should the GM do? Which of the three options (or any additional possibilities) should he pick? How should that be determined?