Mail Help

How do you get rid of an email account? I was sending out an email to a friend and I just started typing her email address into the "to" area of the email. It automatically trys to find her email address. She has two different ones in my mail account...one is her old work place (where she used to work...now her old boss gets her emails coming in) and the other is her new work email address. I wasn't paying attention and it went to her old work email address. Now I want to get rid of her old email address so her old boss doesn't get her emails. How do I do that? I deleted her old email address from my address book but mail has memory of her old email address too.
I hope you understood all that and hopefully someone could help me. Thanks in advance!