How to synch multiple calendars on iPhone 4s?

Hi All, I work for a large corp. in I.T, I am wondering if there is a way to synchronise multiple Outlook Calendars with the iPhone 4s, i've setup the mail client with the users email account & set it to synch mail/contacts/calendars, but it only seems to synch the users own calendar. For example, one of my users is an executive assistant, and has her executive's calendar available in Outlook, She would like to be able to view & make changes to her executive's calendar on her iPhone. Any help anyone can provide would be greatly appreciated.