Change Outlook Default Calendar

One of my bosses has a public calendar that he shares with his secretary. She makes his appointments, so this is a very convenient setup. The public calendar is the one he always refers to for all of his appointments and meetings. The problem ariseswhen he gets an invite, outlook puts the info into his default calendar that comes with outlook but not into his public/shared calendar. How can this problem be fixed so that invites are automatically added to his public (shared) calendar. Is there a solution? Please help!

Thanks