What does the quote "treat as a friend who treats you as a friend, treat your master as an enemy" in the novel God's Bits of Wood mean?

The directive to "treat as a friend who treats you as a friend, treat your master as an enemy" is based upon an implicit assumption that someone who treats you as a friend must treat you as an equal, not as a slave or servant. If someone treats you as if they are your master, they are not your friend, since being treated as an equal is of the essence of friendship. This quoted passage, then, conveys the advice that friends should be treated as friends and that enemies should be treated as enemies. This represents a particular understanding of justice—one definition of which is treating people as they deserve to be treated—and a particular understanding of friendship, which to the extent that it is grounded in unjust master-slave relationships, is not really friendship at all.