just a small fix to allow also ddm's to be highlighted when the field has an error:
in flexigrid-add.js and flexigrid-edit.js look for the line:
$('input[name='+index+']').addClass('field_error');
and just after it add a new line:
$('select[name='+index+']').parent().find('a').addClass('field_error');
in flexigrid.css change:
input.field_error
to
input.field_error, a.field_error
now when for example you have a dropdown with an error on the validation it will also get the red border.