The study session drags on. Rosier makes sure to take every chance he can get to rib them about their lack of magical heritage. Dorcas, in turn, ribs him for having access to magic all his life yet only achieving average marks. Lily and Severus, for their part, make as many references to muggle things as they can, knowing the bigoted boy won't be able to understand and is much too prideful to ask.
It's a long study session.
Rosier, as it turns out, is a surprisingly competent tutor. Despite all the insults, he manages to explain the intricacies of the Wizarding World in great detail, while also keeping it easy to understand.
Maybe she'll steer him towards teaching. It would probably keep him out of Voldemort's hands, as Dumbledore usually prefers his professors to be Dark Mark free, with some notable exceptions.
Some time later, Severus makes a dry comment about Slughorn—something about what he'd lost first: his hair or his dignity—that makes the rest of them giggle. Rosier included. The boy stills after that, as if realizing that he'd just laughed, that he'd genuinely enjoyed himself (if only for a moment) in the presence of not one, but two mudbloods.
He packs up his things and leaves without a word.
The atmosphere is different after that.