If he can connect locally, and you cannot connect remotely it is one of three things:
- Faulty or missing port forwarding
- Faulty or missing port in MineOS firewall
- missing https or :8443 in the URL you are using.
You need to use the URL “https://[your friends external IP adress]:8443”. Please observe the s after http. This is not added automatically, and must be there.
To access the WebUI port 8443 (in addition to the normal Minecraft port) needs to be forwarded in your friends router, since the WebUI uses that port and HTTPS for connections.
For firewall settings, please direct your friend to: https://minecraft.codeemo.com/mineoswiki/index.php?title=Iptables