So before a few months, I was super excited to create a server and play with my friend. My friend was joining successfully but after a few seconds, he was getting kicked out with an error. I posted a question on this forum only to find out that my upload speed wasn't enough. Now that I decided to have a better look into this, I came across a few sites that say that 1Mbps upload speed is enough for up to 3 players.
Wow. you might wanna upgrade your router or pc network card soon but, It would prob be possible just a huge pain. I'd run paper spigot for best performance but even that might not help. I would honestly just buy a server host or ask a friend to host it for you, also what was your friends error message because it could be unrelated.
That thread that you referenced is nine years old, when Minecraft was still in beta. Minecraft is a much more resource-demanding game now then it was back then. I would say that 1MB is probably not enough. The best way to test that though would be to actually run a server and see what kind of lag, if any, other players have when joining. As you said, your friend was having trouble staying connected, so I would say that your connection is likely to blame. Like Mason said though, what error message is your friend seeing?