Not sure but think they just wants the cache to be rebuild not the actual templates to be reverted... Because they have edited the admin template in the database instead of using the template editor...You can import the master style again by visiting /install/ and clicking Rebuild Master Data. That will restore all original admin templates and ensure the cache is updated.
edit - note that the admin templates don't have a revert option like the regular templates do. You are editing the master when you edit an admin template. Importing the master data overwrites those changes to the admin templates.
Enable debug mode (put $config['debug'] = true; in config.php)@Cezz yeah exactly I don't want the templates reverted. In regards to your question, I can't see the admin templates in the ACP unless I'm not looking in the right area. I went to DB as it's the only place I could find to edit the ACP templates.