Thank you B00 for supplying that answer. All registrations are connected to one's email address - so no two people can share an account with two different usernames. If you want to be extra safe, I'd also suggest you register an account under an alternative email address but keep the username and password (for both the email account and the AB account) for your records.
If you would still like her to use your email address, drop me an email through the Contact Us page, and I can un-register you.