He can't see anything? He can't even recognise that there is writing there?

Yeah, i'd be interested to see if anyone has an answer for this. I wear glasses for long distances and while,unlike your friend, i can see the writing is there, I have noticed that writing in red or green is significantly harder to read even with my glasses on.

I have this problem. With me its red text in emails. Where I used to work they were continually highlighting the most important parts of emails in red. I had to edit my emails all the time to change the text color, it was very annoying. I spoke to my optician about it and he said it was a problem with my eyes not being able to distinguish well between different color brightnesses, brought on by eyes not focusing well and an eye condition I have called Fuchs dystrophy which means I see more of a halo around bright lights than other people do. Another thing I have found out though is that different colored lights focus differently in glasses. Red, green and blue light have slightly different refractive indexes. This means that You cant make glasses that perfectly focus all the wavelengths you see. Most people don't notice this but maybe if this is combined with various eye conditions it can add up.