My friend is getting this error on installing the game mod, we already checked the firewall config and the game version.
If the game version was wrong, it would result in a different version. So no need to worry about that.
This error is certainly a connectivity issue and can be caused by various reasons, such as the Firewall you mentioned.
If they haven't already, they could try restarting the computer which will reset the network adapter and this can resolve the issue as well in some cases.
If that doesn't work, check the use of a VPN. Having one enabled can cause this, yet at the same time using one can also fix this. Often the issue is a communication problem between the client (PC) and regional update server.