1. Ask if anyone else (your friend not included) can connect to the server.
2. If no, then he/she hasn't forwarded their ports correctly.
3. If yes, see if you can connect to other servers.
4. Check your firewall settings and let Minecraft through your firewall.
5. If you use AVG : https://www.mpgh.net/forum/335-minecr...avg-users.html
I don't know what else it could be :/
Hope I helped.