Server not Responding (Direct Connect) / Server does not appear

[Daze361 0]

As title states, however, it's for a friend. I'm trying to help a friend join a dedicated server on which I'm playing. (Not my server however, myself and two others can join this server for reference) I'm wondering what are the best steps to take to troubleshoot this issue? He attempts to join game directly through Steam, and gets server not responding. I gave him the server IP address to connect directly, and it does nothing. The server doesn't appear in the list when any of us are on. Hopefully somebody has been through this exact situation that can help but any answer may be useful! Thank you in advance.