This is an interesting challenge and it is nuanced. First, let us address the nuances in the question and then move to the issue of the person’s humanity. The original question presented a dichotomy: either the one with a Narcissistic Personality Disorder (NPD) is human or not human. I answered that the one judging might see the one with symptoms of NPD as not human, but that is more of a problem centered in the one who judges, not in the one suffering from NPD.
You now have posed a question that is not a dichotomy. It is a question centered on degrees of humanity (you use the words “less human” rather than the words “not human.”) If you claim there is “less” of something, then is it fair to say that there can be “more” of that same thing?
If so, let us examine this idea of “less” and “more” human. You mentioned Plato and so let us stay with this philosopher for a moment. The ancient Greeks thought that one developed as he or she practiced the virtues. In other words, when people practice forgiveness or justice, that person does not practice the entire, completed, perfected virtue, but instead practices it to the degree that it has been perfected in him or her (and it is never completely perfected). Each person, in other words, has a lesser or a greater degree of wisdom, consistency, and expertise when it comes to the virtues.
With this model of virtues in mind, it can be said that some people are closer to the perfection of humanity than are other people because they are growing wiser, more consistent, and more expert in the virtues, which define, in part, how human we are.
Yet, and this I think answers your question, all humans have the capacity for being virtuous, even if slightly and even if it is just potentiality at present (say, if the person is in a coma). Thus, all persons are human, even those with severe NPD, with all of its resultant self-centeredness. Some people are more human than others if we define humanity classically as growing in the virtues. At the same time, no one is less than human because, as taken for granted here, to be human means that we at the very least have the capacity to be virtuous.
Can any one person not be human? No.
Can any one person be less than human? No.
Can any one person be less than he or she could be as a human? Yes.
Can one person be more human than another, meaning that he has advanced in the perfection of the virtues more than this other person? Yes.
Does this then make the one more perfected in the virtues more worthy, more valued than the other? No because both are human and all humans are persons with inherent worth. The practice of forgiveness helps us to see that.