Is this users mailbox still present on the Exchange server and is the account still present in AD?
If this is the case then you can do two things:
1.Use Exmerge (2gb limit) and extract pst of this user and then import the same into a seperate folder into the boss's Outlook
In the below link start with the section "Configuring EXMERGE"
http://www.petri.co.il/brick_level_backup_of_mailboxes_by_using_exmerge.htm
2.Grant Full mailbox access on this users mailbox to the boss's account so that the boss can open this users mailbox in his own Outlook profile
http://technet.microsoft.com/en-us/library/aa998707(EXCHG.65).aspx
If in case the mailbox is no longer present in Exchange and has been purged then you would need to restore from a backup to the RSG
Restore the backup to RSG and then use Exmerge
http://www.msexchange.org/tutorials/ExMerge-Recover-Mailbox.html