Legion is correct. It is an issue with the font where it uses the spacing of the correct letter but displays the letter two positions down from it.
Here is a better description of the issue https://stackoverflow.com/questions/66747171/why-javafx-application-and-scene-builder-is-showing-garbled-text
My FaF client is in a different language
Have you tried uninstalling, rebooting and then reinstalling as I suggested? Maybe removing the specific font and then reinstalling could also help but I have no idea what it is called.
If the problem is that a font was not installed correctly, can he just install that font, reboot the computer, and then it would work?
Can someone just send him a zip file with the font that he can drop into the windows/fonts directory and that would fix it?
It doesn't use a font that is installed with windows. It uses a font that is packaged with the client and placed inside the client's installation directory.
Or set a different font (one that comes with windows) in a CSS customization file. Where would that go again?