I think the first thing would be to try to establish where, or what, was causing the problem eg
- Your Co-owners computer/network
- His ISP
- The Web server
- The Website
Since it is possible to have an IP blocked at the server for a variety of reasons that may include repeated
FAILED logins to cPanel, Webmail, FTP, SSH, POP3/IMAP/SMTP or authentication to a protected web directory, and because you have reached out to the cPanel forums, we have to assume that you think the issue stems from the server - these are some things I would check to start to diagnose the problem:
1) Can your Co-owner connect to any other sites on the same IP as your site is hosted on?
If no, ask your hosting provider to see if your co-owners IP is blocked in any firewall or other security device that might be active on the server.
If yes, check to see that your website has not added his IP to any sort of security plugin or htaccess restriction.
2) Can your co-owner connect to the cPanel for the hosting account ?
Check to see if his IP was inadvertently added to the IP Blocker list
3) Check to see if your server IP is listed on any RBL eg try
http://www.anti-abuse.org/multi-rbl-check/
If yes, you will have to ask your hosting provider to get it removed.
4) Try and change the Name Servers your co-owner is using in the network connection for his computer.
5) Flush the DNS cache on your co-owners computer and see if he can then connect
6) Check to see if any security software running on your co-owners computer has quarantined or blacklisted the website or its IP address for any reason.
7) Uninstall and re-install any network adapters on your co-owners computer - make sure any adapter software is up-to-date.
8) Revise any modem/router DHCP and auto Nameserver and wifi (if used) settings
9) Complain to the ISP which most likely will start a
Blame Game with the ISP blaming the Hosting Provider blaming the user blaming the..... you get the idea
10) Take your co-owners computer to a different location that is known to be able to connect, and try from there (eg if you can connect, try his computer at your house, or conversely your computer at his house)
Hope something helps.