[2021] Friend can't join any games but can host his own

I was thinking he must have Steam ports blocked by a firewall somewhere, but the stellaris_test branch uses different ports so that seems unlikely.

Has this always happened for him with this game? Or it was okay up to some point?

Does it also affect our similar games CK2, EU4 or HOI4? CK2 is free to play on Steam now FYI.

What happens when he tried to join, any error?

Any mods involved here?

Can he try from a different machine, either on the same network or elsewhere, as a test?  

Then it seems clearly an issue with his computer, local network or ISP. Lacking the ability to try different computers or networks I can't really see how to narrow it down further, I'm sorry. They definitely can't install the games on another machine elsewhere, say another friend or relative, as a test?