*flips back to the page* I see what you mean, the dialogue works. But the "According to Ron" still sticks out as a definite assertion of fact, whereas it would have been more sporting to the reader to have had a description of Ginny looking upset and asking questions about how Mrs Norris was recovering, followed by Ron saying "Look, Ginny, I know you're an animal-lover, but Mrs Norris really was a nasty bit of work and we're honestly better off without her." That way we'd have Ginny's exact questions, followed by Ron's assumed interpretation of the motivations behind them.

Would we readers have guessed right off that Ginny was responsible, given that extra expansion of the text? I doubt it: we'd have been on the lookout for a consciously evil perpetrator, not for an innocent little girl controlled to do these things against her will. Rowling could have afforded to give a direct clue here, imho.