I set up a Minecraft server the other day, and I have successfully set up many in the past. It is properly port-forwarded and I can confirm via. port checker tools that the port 25565 is open on my IP when I launch my server. All my friends and I can join, except for one friend. He can join other distinguished servers, such as Hypixel and Mineplex, but he cannot join mine (but we have not tested if he can join other peoples' port forwarded servers). He cannot connect to my IP on his end through this port, and nothing in any way signifies that it is open for him. He is certainly using the correct IP; I've drilled him about that. I have tried to lower my firewall and he has disabled his firewall in an attempt to join, but this has yielded no results.
He is using Optimumwifi and windows 10. He has Windows Firewall, but again I doubt this is an issue because he has attempted to disable it.