I am posting this here so I can hopefully get some help because I am completely stumped by this issue that a member of my forum posted. Basically it seems like FontAwesome is not getting loaded for this particular user which is leading the icons to display as ugly squares.
It's also possible to cause issues with browser extensions that allow a user to inject their own custom CSS, that happened to a user here, recently. They were overriding the font family universally for the entire site.
Also the actual browser version could be factor. Specifically Internet Explorer 7, or new versions of Internet Explorer with Compatibility View enabled, could cause this. I would guess that old versions of Opera, Firefox and Chrome could cause problems, but that's less likely. Either way, let us know what browser extensions they have, if it works with them disabled, and the specific browser and version.