So you are saying that no one in the company ever sent this person an email? I don't believe that. Something will be in someone's sent email.
Failing that you probably have some kind of email template that makes guessing email addresses easy.
Find out what the persons email address was and add it as an alias to your account and then do a password recovery.
Trying also using any common email addresses in your company like help@, support@, administrator@.