Well, he'd hate for the OPs friend to see my son. My son dresses like every other teen I know and he loves rap music. He doesn't use correct English when he's hanging out with his friends.
He'd die before he'd wear an outfit from A&F.

My sister and BIL said I might as well paint a bulls eye on his back.

He wears Sean John, Akademiks, Roca Wear, Phat Farm... Yes, his pants MUST sag. He wears a belt, but you'll never see it. The only time he tucks his shirt in is for church and school. As soon as he steps out of the school building, the shirt comes out of the pants.
He (and many of his friends) took the SATs in the 7th grade for the John Hopkins Talented Youth Program. They all scored high enough to attend the Talented Youth Summer Programs.
My son is also an honor student. All of his teachers recommended him for AP classes. His AP interview was last week.
He's well mannered and respectful.
I hear teens and adult using poor English all the time. If I don't know them, I assume they know the difference and are just speaking a certain way because they are relaxing with friends. I choose to do this all the time depending on whom I'm around.
If I hear people using poor English at work or in a classroom, I just shake my head. That tells me that this person doesn't know when or how to use correct English. (Or they just don't care)