Hey angusjt, do you want to PM me the server IP that your friend gave you and I can check it out?
Generally speaking the following is required to set up a minecraft server to allow access from outside sources:
1. Find your public IP using whatsmyip.com
2. Set a static local IP for the PC/Server where the minecraft server will be hosted
3. Set up a port forwarding rule in the router/modem to point any incoming connections to port x to the local static IP and port of the server/pc where the minecraft server is hosted
4. Edit the server.properties file and set server-ip and server-port to the local static IP of the server/pc hosting the minecraft server (the server port for the minecraft server needs to be the same here as it is set in the forward rule in the modem/router
5. Create a firewall rule on the server/pc to allow connections to the port you have chosen to host your minecraft server on
You would then use the public IP that your friend sees when he visits the website in step 1 and the port your friend chose to use for the minecraft server to connect to it in the format of ip:port e.g. 192.168.1.0:25565
Additionally depending on the router/modem your friend may need to set up a static route so that the modem/router is 'aware' of the PC/server that the minecraft server will be hosted on.
I would recommend checking with your friend to make sure that he completed these basic steps. Ultimately if the IP/hostname does not resolve then it's a problem with it not being publicly visible and this leads me to believe that one of the above steps was missed.