I feel like this person is kind of missing the point of cultural appropriation. Yeah, black people from poor neighbourhoods are absolutely more persecuted, but dreadlocks don't only come from those neighbourhoods - they come from a cultural history that you would share with them. Not to mention that frequently people assume black people are lower / working class just because they are black anyways, and having dreadlocks is sometimes seen as a marker of this (which is why white people are so invested in thinking about them as "unclean"). Dreadlocks and other hairstyles that are traditional and natural for afro-textured hair have been portrayed as "unclean" for years and are currently still heavily policed as being "unprofessional" in many contexts. This obviously doesn't only happen to poor black people. Hell, wasn't there a story in the news not that long ago about a black man who got hauled into jail after buying an expensive belt or something? He had already paid for it and left the store, but because he was black they assumed he had stolen it. That had nothing to do with his class and everything to do with the fact that he was a black person.

It makes absolutely no sense to say that only those who are the most persecuted for practicing their culture are allowed to practice their culture - especially when it's someone who isn't even from that culture saying so. Your aquaintance is just going right back to policing black people for how they "do" blackness to say this sort of thing. The same idea that says you are somehow "less black" because you are middle class is what lets people do weird shit like decide that some black / other racialized people are "the good ones" because they can perform whiteness in certain ways. And then, consequently, you say that the only "really" black people must be "the bad ones," further enforcing the idea that being black is inherently bad / unclean / immoral.

tl;dr it's really fucked up for a white guy to claim to know more about black culture than a black person. also class and race are different things. also while i am sure there are differences between poor black [insert wherever you live here] culture and middle class black [insert wherever you live here] culture, dreadlocks aren't exclusive to one particular group of black people as far as I'm aware since they are a pretty natural way of wearing hair of a particular texture. also as an also white person who does anthropology, athropology's entire history is racist as fuck, so if your acquaintance is trying to use that to tell you they know your culture better than you do please don't hesitate to tell them they are full of it and need to go back to their books.