I knew one other adult who could not read. He was VERY good at hiding the fact.
The friend when I was younger was white; the most shocking thing to me was working with blacks with college degrees who would have never gotten out of grammar school with their English skills (I know there are whites like this too, but not at my job).
You don’t realize that they spell phonetically how they speak; it is really sad, and a real disservice was done to them by passing them. With more effort earlier they’d be much more today.