I'm not sure whether you can load books up on an unregistered Nook, but for a registered one you're supposed to be able to put books from different accounts on it, as long as you've got the Name/CC# info for the account from which the books originate.
A couple of Nook owners on this forum have done it with success, I think, and hopefully will pop up to confirm/deny.
You might also consider creating a new B&N account in her name, buying the books under your CC# and registering/loading up the Nook, then removing your CC/billing info and turning over the password to her when you present her with the Nook so she can switch it to her own email address and such.
This is something that's also supposed to work, and there's
another thread right now in this forum about it.