Cannot hide "with whom" column in overview

Hi,

Using Scheduler version 2012102800, it is impossible to hide column "With whom" in the Overview tab. I tracked down the problem to https://github.com/bostelm/moodle-mod_scheduler/blob/master/datelist.php#L117, where the column 'CONCAT(studentlastname,studentfirstname)' being discriminated. I just change it to 'Whom' and it work.