This is why good managers don't aim to be friends with the people they manage. You may end up being friends because of team camaraderie and the amount of time you spend in the same building, but deliberately choosing to socialise with some (but not all) of the team is a bad idea.
And exactly the same thing happens if there is a perceived friendship in the team. Was the employee promoted because of their skills or because they're mates with the boss? Were they not promoted because the boss didn't want to be seen to be promoting a friend? Did they get into that argument because they're a brown-nosing jerk that thinks they're untouchable because they go for beers with the boss every Friday?