Outlook also caches email addresses that you type in the TO box (ie. they
aren't stored in any address book - they appear as you start typing their
address in the TO box). Maybe this is what you are referring to?
However, the address book is irrelevant for when people cannot send you an
email. Is she the only person that cannot send to you? Are you receiving
emails from other sources? Do you have antispam software that is designating
all her emails as spam? Are her emails being classifed as Junk Email and
sitting in the Junk Email folder?