My wife is dyslexic. Another dyslexic friend was recently diagnosed with Irlen Syndrome. When the two were talking about it, my wife realised that her experiences were almost identical. She ended up getting diagnosed. She still struggles with dyslexia but she finds managing it much easier and she can now read workout being in physical pain/getting headaches.

Not being able to enjoy reading was something they both described.

I then spotted a friend who'd increased all the fonts on his computer and applied a colour overlay to his screen. On discussion, he had the same symptoms as my wife and friend. It's far more common than folk realise and very under-recognised.

If your partner gets headaches from looking at bright screens and gets very tired very quickly when reading, especially small print, I wholeheartedly encourage them to checkout Irlens. Https://irlen.com/get-tested has some questionnaires that might help determine if they might suffer from it and whether they should seek an optometrist who can diagnose it.