If you know her email, make her a bn account if she doesnt have one. Then go and get a reloadable debit card from a store and set it as the default card, that way she can only spemd what you put on it. Then just turn her loose lol.
I did this for my daughter once.
Get an email account at gmail, or yahoo to use to register the device. The recipient can then change the address without unregistering after they recieve it.
To add books to it that you purchased, you can sideload the books onto the nook, open one to get the pop up that asks for the information for the account that purchased the book. Put the information (cc number and your email addy) in once and all the books can be opened and read.
Future purchases can not use that information, but require the new owner to use their own cc to purchase ebooks. They can use the same email address that you set up for them or change the email address in their account to their own. It doesn't require them to deregister the device, just change the email address associated with the device.
THis way, you can set everything up for them without the person knowing about it or compromising your own information.
ANother option to load the nook, is to purchase one of the visa gift cards. Give it to the person to use to purchase items or use it yourself to purchase apps and ebooks for the nook. When the recipient gets their nook and puts in their own credit card information, that information will transfer to the books next time they are downloaed to the nook.
You could also get her a Nook and a B&N gift card. She'd have to set her Nook up on her own, but then once she enters the B&N gift card her purchases will come off the card first (rather than out of her own account).