Oh, this is beautiful, and it left me near tears both the first time I read it and right now when I was "reviewing" (*cough* rereading closely) it in order to comment.
Never drink alone. Never drink when you're angry. Rules to live by--the elder Lupin likely trying to prevent the alcoholism that a lonely, scarred young man who feels like a perpetual outsider could so easily slip into.
I really like how you kept a bit of clinical distance between them, these men who come to rely on one another but who won't say it aloud. Two men, scarred from battle...it's terribly ironic that Remus doesn't consider how much more of a reason Mad-Eye has to be self-conscious about his looks (even as Remus observes the puffy skin abutting Mad-Eye's prosthesis). But oh, of course he doesn't. Moody, with his dictionary of silences, is the stalwart to Lupin's insecurities.
And that last scene: heartbreaking. Remus has lost not just his comrade and erstwhile lover, but the man with whom he knows how to grieve. It guts me.