I have a question regarding the extent of GM authority. I would like people to answer this poll to see what the gaming community thinks should happen in a particular situation.
What are we playing? Because there is seriously a lot of difference between games. And an elf is not out of line with the pitch of "D&D regular people, game of thrones".
If we are playing D&D or another explicitly kitchen sink fantasy game then I seriously question whether Sir Knightliest of the Knights and Lady Nobliest of the Nobles are any more "regular people" than the elf and I wonder which the DM is objecting to (and whether the elf is a wood elf or a high elf). And we have a case of the DM having decided that in their fantasy Game-of-Thrones, despite being D&D it's not just all races on the continent that are human it's the entire world. The DM clearly isn't flexible enough to even cope with the idea of a visiting merchant or ambassador (or, more likely, ambassador's secretary)* and the game is therefore going to be ... frustrating. Neither the player nor the GM come out well at all but the GM loses hard.
If we are playing Apocalypse World the players all ask the MC why the hell they started world building before Session Zero anyway.
If we're playing WFRP the big question is "which type of elf and what are they doing there? And are they anything to do with the Wood Elves setting themselves up as the Lady of the Lake?"
If we're playing Fate, again the question is "what does being an elf mean?" Strange foreigner from far away is one thing, ambassador of the Seelie court is another.
If the pitch is Mechwarrior with court politics I wonder what planet the player is from when they want to be an elf. Here I'm on the DM's side - as I am in any system that does not by default include elves.
The game matters.
* If the pitch is either "Fantasy Londo Molari or, better yet, Fantasy Vir Koto dropped into this place on the far side of the world to keep them out of trouble" I'm immediately on the player's side. Vir Koto is
absolutely a regular person dropped into a Game of Thrones-like situation.