I think you've stumbled upon one of the most important and difficult cultural questions of our generation. I don't claim to have a robust theory, but I'll throw out an idea:

4chan becomes a place to say edgy racist things in order to get a rise out of people because it's amusing. Most people don't actually believe the racist things they say, but now saying racist things is normalized on the site, and it becomes more attractive to people who want to say racist things because they believe them and not just to get a rise out of people. Now it becomes very hard to tell who's an edgelord and who's a skinhead. When people develop friendships within this community, they aren't able to filter out the skinheads because they are indistinguishable from the edgelords.

First, skinheads' arguments sound a lot more convincing when they're coming from your friend's mouth as genuine professions of belief. Second, no edgelord wants to admit that someone else is too edgy for them, so there's social pressure for everyone in the friend group to adopt the beliefs of the most politically extreme member.