If your mail client can't do it then yoi can do it through the websieve interface at http://imap.<mydomain>/cgi-bin/websieve.pl using her username and password

You will then have to subscribe to those mailboxes under the "other users" hierarchy to see them (right click on your folders, and select 'IMAP Folders' if you use Outlook / Outlook Express)

The UGLY way is to add her account as an extra account in your email. This has one advantage in that you can reply 'as her,' but will considerably increase the system load