I'm playing devils advocate here a bit, but just suppose you were to rule this the way I do? The players who die still receive full exp for the boss battle. They took one for the team, but they don't get punished for it. Instead, the whole team is rewarded equally for their group accomplishment. While waiting to be resurrected, the player who died temporarily takes control of an npc,
OK, while I'm not entirely on board I can sort of see where you're going up to here...
but any exp earned by this npc will eventually go to their character.
...but at this point you completely lose me again. Experience earned by an NPC stays with the NPC, who is every bit as much a levelled experience-earning entity within the setting as a PC is. As far as the game world is concerned there is no difference between them...characters living in the game world don't have little stamps on their foreheads indicating whether they have a player attached or not, and nor should they. Or does the NPC "double-dip", earning experience both for itself and for the dead PC?
After they've been resurrected, their character receives all of the experience that it would otherwise have missed out on. No sour grapes, the team is rewarded collectively and fairly.
Which, in effect, means the PC gets experience while it is dead; a rather absurd notion* to begin with and truly ridiculous if taken anywhere beyond a brief death-raise cycle.
What happens if a character dies and for whatever reason isn't brought back for quite some time - say, two or three adventures - and the player rolls up and runs a different character which retires (but is still out there) when the first one is revived? What if the player goes through two or three characters during this time?
What happens if the Stand-in NPC (SPC?) manages to lose a level during this time; does it drag the dead PC down with it? Ditto if the SPC manages to gain a bonus level e.g. via a Deck of Many Things?
What happens if the SPC dies?
Simply put, my character doesn't deserve to earn xp for things that happen while she's dead. It's not like she's actually doing anything* in an adventuring sense, and any sour grapes regarding fairness would quite logically go the other way: why should my character get xp for doing nothing while the rest of the party is doing all the work and taking all the risk?
* - one very rare exception to this IME has been the occasional time a character has somehow managed to do some adventuring on its own
while dead in such a way as to be able to retain these memories in form of xp once revived. Depends how much one-on-one mini-dungeoning the DM is willing to do in the lands of the dead.
Lan-"this dead guy on the travois isn't pulling his weight - in fact, we're pulling his weight"-efan