I don't know what port CorelDraw uses. But it very well could be blocked by the firewall. A quick test would be to disable Windows Firewall from the Services applet in Control Panel. If it starts working right away that was the issue. Then you need to find out which port it tries to use. Probably could find out from a forum on the Corel web site.
Make sure the customer enables the firewall after the test so you won't be blamed for any malware.