function _get_user_account_id() {
  global $user;
  return db_query("SELECT `field_account_target_id` FROM {field_data_field_account} where `entity_id`=:user_id and `bundle` = 'user'", array('user_id' => $user->uid))->fetchField();
}

function custom_node_access($node, $op, $account) {
  $account_id = _get_user_account_id();
  if ($op == 'view' || $op == 'update') {
    if ($node->field_account['und'][0]['target_id'] != $account_id) {
      return NODE_ACCESS_DENY;
    }
  }
  return NODE_ACCESS_ALLOW;
}