In Complete Liberty podcast 185, Heiko discussed descriptive ethics and how to reframe NVC as not entailing moral relativism. It is a bit long, the discussion gets interesting about minute 50. Unfortunately I must admit I can't summarize the idea that has me so excited, just describe it. So while I found the discussion very interesting, I did not find it clear, even on the second time through. But hey, I was walking the dogs while listening, maybe I need to sit down and concentrate.
NVC labels moralistic judgements as a form of communication that blocks compassion. Because we need to give ourselves empathy, this seems to entail a moral relativism, where judging in terms of morality is just off the table. But this itself is contradictory, at least by one reasonable interpretation, because NVC can be seen as implying ethical rules of behavior itself, and making moralistic judgements against certain forms of communication and thought.
I think the resolution of the contradiction lies in distinguishing between what thoughts, feelings, needs, and requests a communication expresses, and whether it does so in a way that provokes the listener or not. One way of expressing a truth ( e.g. "You are an asshole") may provoke the listener while a different expression of the same idea ("Please don't call me at 2am unless it is a dire emergency") has better chances of success. By this interpretation, Rosenberg is not embracing relativism, but pointing out that moralistic judgements tend to make people defensive, and even if they succeed in getting the other person to change their behavior, there will be a bill to be paid in the coin of resentment.