I disagree, but you do need to keep things in perspective.
I would put the brakes on a player dictating too much about an NPC that is going to have any lasting impact on the overall game/story. If the player wants to specifiy things about the tavern wench in the town they are passing through, that is probably OK. Unless of course the player wants the wench to be the secret contact for the thieves guild that has completely fallen for the PC and will gladly do anything to win the PCs favor.
Expounding on a relatively trivial NPC to emphasize characterization and create interesting side aspects of the character is just fine.
Trying to dictate how an important NPC will fit into the game world and story without some serious discussion with the DM won't fly with me.
If the player is so attached to the NPC that the GM can't use the NPC in the way you see fit, then the NPC probably shouldn't be included in any story elements crucial to the game. I would approach it that the NPC remains a background fixture. Like the wallpaper. Nice to have established, but there is no action there.
Explain this to the player. If she wants to dictate how the NPC functions in the game, then the scope of that function can only affect the PC on a personal level. There will be no story attachments to that NPC. There will be no mechanical benefits to that NPC. The advantage is that there will be no disadvantages to that NPC.
Of course, that peels off a lot of interesting interplay and potential story involving the NPC and the PC background, but that is the tradeoff.
In my long running campaign, that I play in, we have included stories about a variety of NPCs. I have written several stories myself!
We have created unimportant NPCs in many ways. Kids in the street we have interacted with and that sort of thing. Established stories of charity and compassion that say a lot about the character without having to worry exactly what the stats for 'street urchin #3' has.
We have also asked the DM if we could include established NPCs in stories. We have mentored and tutored guild apprentices, we have bantered back and forth with displaced aristocrats, we have commissioned items from recurring shopkeepers, you probably get the idea. Again, we rarely have a solid idea on the NPC stats, but we have some idea of background and personality. As I said, they are established NPCs and we see ways we can build an interesting interaction off those NPCs. These are NPCs that we care about. But they are already established and we work to remain true to what has been established while enhancing the NPC in ways our DM didn't put energy into working on.
We have also approached the DM with requests to interact with a specific type of NPC. I once knew I would not be at a session and I wanted to establish an in-game reason why my character would not be available. I asked the DM what kind of NPC might be around that I could 'deal with'. His response was (to paraphrase) 'The guy that arrived in town yesterday is an arrogant sorceror with an entourage. He isn't too high of level, but he isn't a slouch either. He has apprentices with him. He won't have any impact on the game ever again. Have fun.' And I did have fun. I established somebody that I could face down in a social situation that would take up time so my character couldn't join the rest of the group. The reward in the 'encounter' was establishing that my character stands up to bullies, could legitimately face down a relatively powerful sorceror, and that the other players enjoyed reading my story. Later the DM threw me a little exp for it as well, but that wasn't where the pleasure of the story came from.
However, there are also some established NPCs in the game that we would never engage without the DM being present to run them. Many, many, many NPCs. Given that we have been playing this campaign for over 6 years and our PCs are 20th level, we have encountered a lot of NPCs. Most of which are solely the DMs domain. In the last session we interacted with at least 5 NPCs that we would never imagine trying to dictate to the DM how they are handled.
Leaving some room for players to work with an NPC can be rewarding in itself. But the player shouldn't dictate how the DM must engage the NPC. Nor should the NPC provide any solid benefit aside from storytelling. If the player wants an NPC with benefits, she should look at using the Leadership feat, or take her chances with NPCs that you, as the DM, are building and running. There is no reason why you can't strike a balance. But as always, communication is key. In the end it is a trust issue. If the player doesn't trust you to run an interesting and fun game with 'her' NPCs, and you don't trust the player to engage with NPCs in a way that is an improvement to the game, you have problems.
Personally I think you have a right to be miffed. A two page write up is a little extensive for an NPC that isn't designed by the DM, or at least with a lot of DM input. As I said earlier, if the player is that attached to the NPC, set the NPC aside. Leave the NPC as wallpaper in the campaign world. No mechanical benefits, no story benefits. But there won't be any drawbacks for the player either.
