I went thru this some years back. Me and my golfing pard had always connected IP to IP because we had "static" IP's that never changed. And then one day my Internet Provider made some hardware changes, and they started using DHCP IP assignments. So every day when I turned on my computer and connected online, I was getting random IP's assigned to me. The problem was the way they had their equipment set up. They had their equipment set up like a private companies in-house network. All of their customers would now be assigned a new DHCP IP number every time they logged on, and they would not open any ports to allow us to get out of their private internal network. So bottom line was that me and my partner could no longer connect IP to IP because my computer was now trapped inside their private network. My only alternative was to pay them an additional 15 dollars a month for a "static" IP. You might have him call his provider and ask if they had made any changes to their network.
I'm betting the problem is with the Win11 operating system. From the nightmares I've read about, they are constantly sending your system updates, and eventually it will cause things to just stop working in the older programs, such as Links 2003...
There is another alternative. Don't laugh, but if you both still have dial-up, you can connect IP to IP. Believe it or not, I still have dial-up available. I've even had to use it a few times when my Provider would have problems. If your friend has been playing for 23 years, I'm betting he remembers that Links plays really well on dial-up.
Good luck,
Dan