Yeah it makes sense in context, I just mean from Roy's view lol. Roy doesn't know anything about Willow, and I'd say just knows of Acacia as a decent person. It'd just seem weird to him (and most people) for Gryffindors to leave one their own who just fainted from nerves alone with Slytherins. But again he doesn't know much. For all he knows they could be best friends.

EDIT: I see most of what I had to say on the matter was already covered, lol. Honestly it was a joke at first but the part about the Gryffindors leaving her seems more and more uncharacteristic the more I think about it.