Jacob Bronowsky, had an essay on this in his book (and TV series) Accent of Man..
he pointed out, as a child, he had mastered Polish. and that his parents, well educated Jews, were able to get out of Poland in the mid thirties, and settle in England.
There he was age 2.5, speaking only Polish.
but by age 5 when his parents moved to US, he hardly spoke any Polish, and now as an adult he doesn't know remember any Polish at all.
But-- a critial point in his life, (age 12 months to 3 years or so) he mastered language..
Yes, humans are hard wired to see in stereo, but if a child has a "lazy eye" (or some other visual problem with a singele eye) and it doesn't get corrected.. the child effectively becomes blind in that eye..
it's not that the eye doesn't "function" -it is that there is a window of opportunity.. and if you don't use it, you lose it.
So too with language.. Hearing children of Deaf parent might learn sign langauge as their first language.. and later pick up a oral language.. and they do fine.. but they might have some difficulties.. learning to hear the difference between R and L has to be done before the age of 4--or it is not going to happen.
In Japanese, it is the difference between Su and Tsu-- these subtle difference can only be taught at a young age.. and then the pathways harden. and the brain is less flexible..
perhaps your friend, as an orphan, never really mastered korean.. and now, he is unable to master english.. he was critically shortchanged, and nothing now can be done to remidy it completely.
reading, unlike language, does not seem to be so hardwired.. it is something some one can learn to do as an adult. speach and language are not.
