/**
 * Implements hook_form_FORM_ID_alter()
 */
function mymodule_form_node_delete_confirm_alter(&$form) {
  $form['reason'] = array(
    '#type' => 'textfield',
    '#title' => 'Причина удаления',
  );
 
  array_unshift($form['#submit'], 'mymodule_form_node_delete_confirm_submit');
}
 
/**
 * Custom submit callback for node_delete_confirm form
 */
function mymodule_form_node_delete_confirm_submit($form, &$form_state) {
  if ($form_state['values']['confirm']) {
    $node_author = user_load($form['#node']->uid);
    drupal_mail('system', 'mail', $node_author->mail, language_default(), array(
      'context' => array(
        'subject' => 'Уведомление об удалении материала',
        'message' => 'Ваш материал "' . $form['#node']->title . '" был удалён. Причина: ' . $form_state['values']['reason'],
      )
    ));
  }
}

Drupal 7