That was just wonderful! Right of the bat, let me just say that I could read that sex scene over and over again. (All right, I did re-read it several times, and I'm sure I will again.) Actually, all the sex scenes. *happy sigh*
You've done such a great job with characterization, and I think these two lines are the perfect examples:
Snape: The fact that he was dead was no excuse to become idle and undisciplined...
Lupin: "Tea in the afterlife," Lupin remarked. "This must be the Englishman's heaven."
I loved watching the slow process of Severus being sort of 'tenderized' by his time alone and his exposure to Lupin. Another excellent example of characterization; these things sort of have to creep up on Severus before he knows he wants them. This is all over a great story, and your giftee is a lucky girl! :-D Marvelous!