Hi Bee, I would look at it like this: as someone who wants to complete an art piece (or pieces), I will look for the necessary supplies until I have everything I need. So if I were in her shoes, I will buy these 50 maps one way or another if I really wanted to complete the project.
In your position, you're selling the maps, and here is someone who wants 50(!) of them. Give her a bulk discount if that's something you actually factored into your business, or you can ask her to purchase them individually if the bulk discount doesn't make sense for you financially. Either way, if you are too standoffish or refuse, she'll just get her reference maps somewhere else... and still potentially create competition for you, and you'll lose out on a really great sale.