Private Dedicated Server - Friend cannot join


I’ve gotten a password protected dedicated server up and running so my friends and I can play. The server is up after following the general procedures in guides online, and I am able to see the server both in the in-game server browser and in the View->Servers section of Steam. I’ve opened 7777, 7778, and 27015 (UDP & TCP), and so has he. I am able to see and join the server with no problem.

He does not see the server in the in-game server browser nor the Steam server browser. Attempting to join via direct connect results in a timeout. Attempting to right-click -> Join Game on my in his friends list results in a “Server is not responding” prompt. He has all of the filters in the in-game browser set to All.

Did I miss a step? What can we do?