How can I link a task to a shared contact list?

I have a user who shares her boss' contacts and needs to be able to link (assign) a contact in her boss' contact list to a task in her task list.  Can this be done.  I am using Exchange.  She has shared his contact list and his task list.  Her task list is their primary task list (he shares hers as well).  She would like to link a task to his contacts.  Even if she uses his tasks (on her computer), she can only link it to her contact list.

Please help.

Thanks!