A client of ours has a problem with this domain.

Domain was registered 9/2004 at namecheap, expires 9/2006.

He can't update his domain (contacts, email forwarding, etc.) because it's locked (registrar-hold).

REGISTRAR-HOLD: what does it exactly means?

does he has direct access to the domain panel ? because he must first unlocked the domain to make the changes...
if he does not have access to domain panel, he must contact the registrar to unlock this...

Another possibility is the account can be suspended for whatever
reason. Since you say it's registered with namecheap, you should
contact them about it.

If no response, contact "enetwork" here.

sounds like your clients domain was placed on hold enom(namecheaps parent registrar). It might of been due to invalid whois or a spam complaint. Contact the reseller and they can contact enom abuse dept to see why it is on hold

or for a chargeback/nonpaid case