There should be consideration given to both sides of this. The campaign is, in a sense, the DM's character. In an ideal world the DM would account for everything and the players won't be able to derail it utterly, but in the real world occasionally something will slip through and I think the players should allow for this kind of whoopsie. Just like if a player who had a deep investment in a character did something that unexpectedly and pointlessly killed their character off, I would be open to giving them "backsies." As long as they were open to it.

Might be worth adding this to the session-zero discussion.

If the idea of Boromir knowing he's been killed but still fights until the very end ISN'T something you want, nobody forces you to my friend. We all have different tastes for when we'll go out.