I'm confused.
The issue is your player wants to use material no one owns, and it's not fair that you don't allow what to you guys doesn't exist?
Your players have chosen to play 4e instead. Do you have the books? Do you know how to run it? If the answer to either of those questions is No, than it's not going to happen successfully.
I personally encourage creativity with my players, and balance it by throwing custom versions of obscure monsters with twisted templates at them, etc. If a player approaches me saying he or she wants to use some material I'm unfamiliar with for the character, my standard response is "Ok, get me the book/Magazine that comes from, leave it with me for a few days to read over and learn about, and I'll get back to you". They either produce the material for me or don't, and I usually approve it after reading (We've banned Unearthed Arcana's modifications of the base classes, didn't want to give my players THAT much freedom). Once I'm familiar with what the player can do, I can tailor the campaign to give them a chance to use the abilities they want, and still keep them challenged.
Do what you are both realistically able to do, and comfortable doing.