How to restrict field permissions to referenced user?
I assume field_reviewer is an user reference field with only a single value allowed. If so use the following to get the referenced user's uid which is directly stored in field_reviewer array in submission:

// is current user == node reviewer?
if (!empty($entity-&gt;field_reviewer)) {
  $reviewerId = $entity-&gt;field_reviewer['uid'];
  return ($reviewerId == $user-&gt;uid);
}
return FALSE;