Well I would suggest the obvious of his password being input wrong...
Seriously though other causes for that error might be improper server address / port being input. You say "one user" does that imply that others are able to connect properly?
If you have proper SRV records setup for your domain pointing to your Zimbra server's XMPP instance then all people should need to do is put in their user name and password and pidgin should grab the server address and pot automatically.