You don't mention anything about the rest of their server.
If everything but /var/opt/scalix/??/s/data was left intact, then they can run omscan, clean up missing children (run omscan over and over until it runs without errors) and sxmboxexp what's left. Impossible to say how much you might get, but you should get some.
If the whole server was trashed, you're sunk. You need the /var/opt/scalix/??/s/user*/g* directories in order to recover data. The g directories hold all the user's containers (inbox, deleted items, etc.). The containers hold pointers to the data store saying what belongs in each container.