It may be impossible to replicate or resolve without direct access to his machine to test on. It may be possible that custom 'large icons' or 'font zoom' settings on some version of windows behaves unexpectedly and outside of the scope of DBU scale, blowing up the font point scale instead.
Just write a custom scaler that doubles everything in your dialog. Add the option: "Jumbo Mode." Or half the point sizes of your font formattings.
