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.
I could never manage the dialect living in Bavaria -- locals regularly asked me why I spoke to them only in hochdeutsch, and I was even accused of being a SauPreuss.... and then I explained I was an American. I actually had to show a couple of people my passport before I convinced them I was not German.