One friend unable to join my servers (multiple games), all others can.

[Kittastrophe]

This began when I made a server for the game Rust. I have 4 friends that were going to join, but only one person of the four is unable to join. I had port forwarded, to my knowledge (and I checked multiple times), correctly. He and I both tried opening the ports in windows firewall, disabling windows firewall, disabling our antivirus/antimalware apps, and some other minor tweaks, without success. When this didn't work we tried to narrow down the problem to see if it possibly had anything to do with Rust specifically. We both created Minecraft servers. Surprisingly, other friends were able to join both my server and his, while only he couldn't join mine. As well, I was able to join his server. Seeing as everyone else could join my server, and the 25565 port appeared open on every port checker website I tested, I definitely port forwarded correctly. It's almost as if something on my end is blocking him only, perhaps by blocking his IP, be it windows firewall, my router's firewall, Malwarebytes, or something else. Any ideas?

 

[Kittastrophe]

MERGED QUESTION
Question from Kittastrophe : "Friends with the same ISP as myself are unable to join my servers (PC Games)"

My friends and I were trying to play the game Rust on a private server which I hosted. I am certain I port forwarded correctly, as I checked multiple times. One friend, of the 3, was unable to join; the other two could join just fine. Both he and I tried opening the ports through windows firewall, disabling windows firewall, and turning off all antivirus/malware apps. I also made sure he port forwarded the ports the exact same way I did. When nothing worked, we tried making a server for other games, namely Minecraft, just to see if the problem had to do with Rust. Same results. Then, he tried making a server for Minecraft, and surprisingly, I could join his server. We couldn't figure this out and gave up. The next day another friend of mine wanted to join our Rust server, but he couldn't join either. Turns out, he, the original friend, and I have the same ISP. Seeing as this could be a problem with the ISP, I called tech support through my provider. The person in support said nothing seemed wrong on my end, and recommended that my friends attempting to join port forward, which of course we already tried, and that if that didn't work turn on DMZ as a last resort; this didn't work either. Seeing as there was no possible way it was my internet's firewall preventing certain friends from the same ISP from joining, and because I can join their servers, it must be something on MY end (my computer) preventing them from joining, right? Any help would be much appreciated, it feels like we've tried everything.