I have a black girlfriend whose use of the English language is extensive (this gal owns the language in ways that astonish me) and very, very precise - no doubt because her father is an English professor.
She says she’s “multilingual”, because she can use the “Black vocabulary” also. She had to learn it though and she uses it to build a bridge to certain people, but she hates it.
She has to watch herself, she corrects people’s language usage out of habit. I don’t mind, some of her black friends are very offended.
I used to do the same thing. Then a girlfriend's father pointed out that correcting other people's language was a sign of low breeding.
I have NEVER done it since. Funny how little events can break you of habits.
I lived it too like your friend, but in Switzerland. Some dialects, like black English and Schweizerdeutch (Swiss German) are bastardizations of the language from whence they came. Hard to go from the correct version to the grammatically “wrong” one, but it really does build those bridges between people. I couldn’t do it in my own language like I did in German, though.