Oh, this was excellent. I love when a relationship is shown like this -- more dialogue would seem superfluous. Just the way they're present with each other makes everything clear.
You've done a really great job painting Lupin's situation, how alone he is, feeling like the last man standing. You feel he's at risk of falling into a very dark place, maybe giving up completely and drinking himself into oblivion. But even though their relationship is brief, Moody somehow pulls him back from the edge. And that doesn't have to be exactly understood or put into words, it just is.