Josh Green is entitled to feel guilty not because he's white, but because he's a member of a group that unjustly voted itself privileges in the area of housing, to the detriment of non-members of that group.
He was definitively not a member of white people in Georgia during the 50's and 60's. He wasn't even born at the time this group lost their special privileges, as noted in the article ("your one-sixth-acre slice of Atlanta had actually belonged to the family next door for longer than you’ve been alive").
Now it might be right for him to feel guilty about not doing enough. But he should certainly feel less guilty than most of us; he's done something, I've done nothing. Never even been to Atlanta. I have many friends who've never been to the US. Should they also feel guilty?
In any case, that's not Copenhagen. Copenhagen says that Josh Green should feel guilty since he observed the situation, but Igor Muscovite (a Russian who never left Moscow) doesn't need to feel guilty since he's didn't make the observation. I guess the best way to avoid guilt is to bury our heads in the sand?