I've had my etsy shop for about four years now and on occasion, I have honored requests from customers to reserve an item. The ball is really in your court-you can set up the conditions w/the customer (how long the item is to be reserved for, etc.) and if acceptable to both of you, it can be beneficial. Typically, what I've done is to change the title of the listing to "Reserved for..." and then your customer can go in and purchase the item at the agreed upon time...from there, the transaction proceeds as any other etsy purchase. Just be clear that you don't want to keep the item reserved indefinitely (unless that's ok with you) and you should be fine.
Reserving items can be a convenience for your customers that may ultimately lead to future sales for your shop. Good luck!
I would first have to ask if she is wanting it reserved for purchase now. Or is what she is really wanting is for you to hold the item?
I sell well on Etsy and I take special orders and do not require payment till the order is ready to ship. At that time I will make a reserved listing and convo the buyer that it is ready for them to go and take care of. And that it will be shipped as soon as it is.