She said that Vulpecula broke her mirror, so she broke one of them in return, and that's fair.
I said it wasn't the same at all, because they didn't mean to hurt her, but she was being malicious... and that it didn't make things even at all, because when Kaito is hurt it's not just Kaito who's hurt, it's everyone who cares about him, too. So by her way of thinking we should all go shatter one of her mirrors in return.