This is a recent (last two weeks) issue for only a single player. He has tried numerous browsers, to include freshly installed ones with basic configurations. I’m unsure what to tell him in an attempt to fix it, as no other players are experiencing it (so its not a configuration on my end).
Any assistance with this issue would be greatly appreciated.
It might be interesting to run through World Size Check - Docs - The Forge to give us some information.
Though, this is usually caused by Accelereation not being properly on, you could try the last couple of steps in How to fix/turn on your browser's hardware acceleration
though, lastly it might be an ISP issue. in this case it usually solves in a day, but it doesn’t seem to be the case. Could you try to ask them to remove IPv6, or try to change your server region?
Another interest might be to ask them to join an new empty world.