It has to be completely rewritten to fix it since the current script uses an API that hasn't been supported by WHM in years. It'll happen eventually. As of now, it still works as long as the new domain is not a .heliohost.us domain (so you can change it to your own domain just fine, you just can't assign yourself one of our subdomains at the moment). Now...I can't change his domain because I can't find his account...
Your .htaccess files are causing this. I renamed them to htaccess.old and its working. Specifically, you disabled directory listings in the .htaccess file, but didn't provide an index.php for those domains. If there's no index and you disable directory listings, you get a forbidden error.