Eventually I created my own server as I couldn't join him (with the same reasons he provided in that topic) and I've been having the same issue. The log is in the spoiler tags.
some log
So the thing is that we can see each other's game in the server browser, we can press that connect button and we're even requested to put in the password. If and only if you put in the correct one, do you get to the next step (downloading map) but it stays at 0 b/ps and eventually disconnects. Before I tried helping this person out with this issue, I have hosted numerous maps over numerous years (All the way back from 0.11 I believe) for numerous friends with 0 issues, even before the UPnP was a thing for Factorio by means of port forwarding (Which isn't required anymore nowadays). As you can see from the logs, there seem to be quite a few errors (non-existing heartbeats, fragments failing, ).His friend could join me without a problem by the way. So it really seems to be some user verification problem he seems to be having? I'll recommend him to make a ticket. He wasn't fond of me adding his IP or username here for personal reasons.