If she's purchased any items from the iTunes Store, select any item and Get Info on it (File menu -> Get Info). It will show the account name there. Then you can use the Password Retrieval page:
https://iforgot.apple.com
to get the password sent to her email account. If she's not purchased anything yet and so you can't look up the account ID, there's a link on that same page to help recover the account ID.
Regards.