If the client is using cPanel, you can use WHM to edit their MX record for their zone (Edit a DNS Zone feature). After you've edited the zone, make sure you remove the domain from /etc/localdomains and place it into /etc/remotedomains
When editing the zone, don't forget to increment the serial, and don't forget your periods. It's a pretty simple task overall, not much to it. If you need more info, just let us know.