I am trying to find a way to import the sendmail backup of one user to another email program so my boss can easily sift through and organize the emails.

Sendmail is only a MTA (Mail transport agent).
All it does it to send outgoing email to the "world" and if it's used on the
incoming side, it will pass mail to another program to deliver it to a user or
store it to a maildir.
But you will never use sendmail to read email, or at least it doesn't make sense to me.

So you need to find your Maildir, which can easily be reused with another reader( Mutt, Kmail etc)
or you need to find out which program manages the mailboxes to import from that one. (Like procmail, courier-imap etc),
but normally they also store everything in a Maildir (or a mailspool file e.g in /var/spool/mail/root etc )somwhere