In general, I don't think names are/should be considered reserved. If there was serious risk of confusion, I might reconsider--but that's confusion on the level of people naming twin boys Christopher and Christian, or Aiden and Jayden (I wish I was making those up, they're from the social security administration website a few years back. In each case, several different sets of parents did this.) Not just "which of my cousins Janet are we talking about?" or "there are three Mikes at this party." (If I was expecting a child, and looking at a family where half of the boys are named Tim and most of the rest are Robert, I wouldn't deal with that by telling other people they couldn't use those names, I would do it by using some other name for my own child, family tradition or no.)
That said, while it's an unreasonable demand, there are also more and less polite ways of making the request, and context might matter. As the OP noted, this is the first she's heard of her friend wanting to call a daughter Charlotte, and the tone seems a bit like "I know you got here first, but how dare you not let me cut in front of you in line for this event." That there isn't a finite number of tickets makes it sillier; it doesn't make it less rude. If her friend had been talking about wanting to name a daughter Charlotte for 15 years, since Charlotte was the 305th-most-popular girl's name in the United States, instead of the 19th, I might have cut her more slack, a gentler "I know you really like that name, but so do a lot of other people. Even if I called my daughter something else, your child would still know other Charlottes."
Your best bet is probably to drop the subject until you send out birth announcements. If your friend pushes, maybe tell her once that there are two people who get a vote here, you and your husband, and that you're not going to discuss it further.