Books are not tied to the device but to your account. Any Nook (color or tablet) will give you access to the books on the account it is registered to.
Deregister the NC and reregister it under a new account for her . Since the books were purchased under your account they will not show up on the NC. You may want require a password to purchase books from the NC so she can not buy books without you knowing it. If you have books on your account that you want her to have you should be able to sideload them and then unlock them with the CC# of the card used to buy them.