If he's on a subdomain http://clientdomain.yourdomain.com
then I think he would need to use a redirect service to have his Domain name point to the url of his subdomain account.
Most registrars such will offer this service I believe. If they don't, then you can do it for free at mydomain.com and then your client will need to change his nameservers of his domain name to point to mydomain.com's nameservers...then in the mydomain.com account, he will then be able to mask forward his domain name to his subdomain url. Hope this helps.