OTOH,his 25 year old daughter...who he brought here as a kid...has a command of English every bit as good as mine and just graduated from Columbia last year.
Native speakers can understand him,with difficulty,but the issue bothers him greatly.
Most of my older students have “fossilized,” which means that no matter what, they continue to make the same pronunciation mistakes that they have made for years. In my classes, I emphasized pronunciation (with these students). It is not the vocabulary or grammar, but simply a pronunciation problem. It is very, very difficult to change. His children don’t correct him, because over time, they understand every word he’s saying.
Your friend can change no matter what age, but it takes dedication and repetition As a teacher, even I noticed that I began to understand their accents and all was well...... while they were in class. They go out into the world, and no one else “gets it.”
Think of all your friends from India. They never improve, but you soon begin to understand every word. It’s difficult for older students.
The fact that English is, in fact, two separate languages, one written and the other spoken, makes it incredibly difficult to master.