It looks like the previous admin has done an upgrade from 4.1.6 to 5.0.2 and also moved from Exchange 2003 to Exchange 2010, but something must not have worked properly.
As he hasn't done a hand-over and you don't seem to have any documentation, I would really start with a new server and install BES.
Make sure you read the documentation before you start:
BlackBerry Enterprise Server Express for Microsoft Exchange - Manuals and Guides