You can change the output through preprocessing, for example in your theme's template.php file, or in your custom module (see also: https://api.drupal.org/api/drupal/modules!system!theme.api.php/function/hook_preprocess_HOOK/7). 

To use the hook_preprocess_block hook, as shown below, the core module "Block" needs to be enabled. 

If used in a module, in yourmodule.info:

    name = Whois Alter Module
    description = A module to change the output of whois.
    core = 7.x
    package = Drupal 7 Misc
    files[] = whois_alter.module
    dependencies[] = block

In whois_alter.module:

    <?php
    // $Id$
    /**
    * @file
    * A module to demonstrate hook_preprocess_block to alter the output of another module
    *
    * This module alters the output of the whois module
    */
 
    /**
    * Implements hook_help().
    */
    function whois_alter_help($path, $arg) {
      if ($path == 'admin/help#whois_alter') {
        return t('A module to alter whois output.');
      }
    }
 
    /**
    * Implements hook_block_info().
    */
 
    function  whois_alter_block_info() {
      $blocks = array();
      $blocks['list_modules'] = array(
          'info' => t(' Alters the output of the whois module'),
          'cache' => DRUPAL_NO_CACHE,
       );
       return $blocks;
    }
 
    /**
    * Implements hook_preprocess_block().
    */
 
    function whois_alter_preprocess_block(&$variables, $hook){
      
      if (isset($variables['elements']['whois_lookup_data'])) {
         $whois_modified = whois_alter_modify_whois($variables['elements']['whois_lookup_data']['#children']);
         $variables['elements']['whois_lookup_data']['#children'] = $whois_modified;
         $variables['content'] = $variables['elements']['whois_lookup_data']['#children'];
      }
    }


    function whois_alter_modify_whois($content) {
       // do what you want to do to modify the output
       return $altered_content;
    }

For Theme preprocessing, just use the functional part of the module in template.php:

    function yourtheme_preprocess_block(&$variables, $hook){
      
      if (isset($variables['elements']['whois_lookup_data'])) {
         $whois_modified = yourtheme_modify_whois($variables['elements']['whois_lookup_data']['#children']);
         $variables['elements']['whois_lookup_data']['#children'] = $whois_modified;
         $variables['content'] = $variables['elements']['whois_lookup_data']['#children'];
      }
    }


    function yourtheme_modify_whois($content) {
       // do what you want to do to modify the output
       return $altered_content;
    }