Field validation is not stopping the form from being send

Hi

I installed commerce kickstart and then i added the field validation module and use it to add a php validation to a field added to The customer billing information profile but it's not working correctly because it didn't prevent the form from submitting it's data

My validation is very simple i want to make a field required only when the user enter a value in another field and for this purpose I'm using this code:

if ((!empty($this->entity->field_authorized_person['und'][0]['value']) && empty($this->entity->field_authorized_person_id_numbe['und'][0]['value'])) || 
    (empty($this->entity->field_authorized_person['und'][0]['value']) && !empty($this->entity->field_authorized_person_id_numbe['und'][0]['value'])) ) {
  $this->set_error();
}

So is there any way to get this code to work?