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My best friend might be a Nazi sympathizer.
A lot of fascism and authoritarianism comes from loneliness or a lack of community. Cutting him off could mean he seeks more community with Nazis. Be clear about your beliefs, keep calling him out when he's wrong, but try to stay his friend. That's different than "supporting" his beliefs. Take care of him when he's sick, but don't drive him to the Nazi rally. Your friendship might make a difference on his journey back to healthy beliefs.
Also, really sorry your friend is dealing with this right now, I know it must suck to see this happen to him. It's not necessarily the final story though.