Mxngox Well there are multiple reasons this could happen.
- how are you connecting? via direct connect (console) or via the serverlist? If direct connect make sure to use the correct ip and port, it's the ip your friend sees when he opens this page: https://www.whatsmyip.org/
- can other ppl connect to the server or is it even listed on the serverlist? If not tell your friend to post a screenshot of his port forwarding settings.