I agree with your casual friend that their are numerous versions of English used in America. In my experience, many “people of color” function well enough in several of these, and jump between different versions depending on the audience.
Other countries have analogous situations. Muslims commonly know the Koran in “Classical Arabic”, but normal spoken Arabic varies so much that people in different regions don’t necessarily understand each other. In Iran, school students are required to learn Farsi as the “standard” language for official use; it is not the most common mother tongue. Similar story for regional forms of Chinese. Students who want to succeed in higher education learn to function in the Standard Language of their country. Only in the US is this an arena for excues and whining.
I agree that we have these different Englishes here also. What I disagree with is that, in our country, we are saying it is ok to not learn “standard English” or use it as an admission criteria, because it’s not what you grew up with. I think it’s important for us to have a common language that we share, and should be educated towards in our youth. We shouldn’t accept illiteracy as a good thing. I do think it’s ok for us to strive to learn each other’s nuances in the language though. Otherwise, we end up like Babel in biblical times.
My grandparents were immigrants from Czechoslovakia and had to learn standard English. They took a lot of pride in becoming Americans, and learning the language was an honor, even if it was difficult for them. It was important to them that they spoke good English grammar. They impressed that (and many other values) on their children, and their grandchildren. Time to pass that along rather than saying it’s not necessary. We live in a very multicultural country. It is good to have a language that unites us.