if your client is using the DNS'ses of a registrar than the best thing to do is setup the forwarding on the DNS servers.
If you're running the DNS for him than you can set it up on your DNS. The advantage is that the browser is redirected on request of the url (ip destination) rather than that he's being redirected when hitting the site as .org and than be redirected to .com.
Any change in the server config than don't have any effect on the redirect.
Hope this helps.