This may or may not be helpful at all because I assume you originally played on community provided servers with your friend;
but this may still be helpful for other individuals in a similar situation.
I know it's stupid that you'd have to do this but if it's better than not being able to play at all, or buying another Steam copy, you could try using a third party application called Hamachi or a similar VPN service. You could use it to host a LAN that both of you connect to and would, in turn, allow you to host an in-game LAN server that would essentially bypass the master server thus no longer requiring key authentication because it's technically not an 'online' game.
I haven't personally used the application in a very long time, but from what I've been able to accomplish in the past it should still work while being fairly user friendly and simple to use.
You aren't just limited to you and your friend, anyone that has access to the LAN should be able to play too. For Hamachi specifically, I don't recall if there's a limit to how many clients you can have or if it all costs money now.
I cannot quite remember exactly how I got it working in the past but I do know it's possible. Heck it might even work right out of the box without any tricky business.
If I had a friend available at the time of typing this, I would've tested the concept again and wrote out a guide of how to do it. I may come back with that but no guarantees (short on friends and short on time hehe)
Best of luck!