Actually, I have a workaround for the kind of situation you used as your example:

Abby: I think it's okay to set cats on fire

Me: You have a very strange definition of 'okay'. (Accompanied by appropriate social signaling that it's not the good kind of strange - depending on the person I may just say outright that I don't approve of it.)

Abby's statement is actually true, assuming she's not joking or being sarcastic or something odd like that. What she thinks is not correct, if she's using a definition of 'okay' that this society would consider normal, but that's not a very good assumption to make in situations like this, and pointing out the incongruity will get farther than trying to argue that a true statement is false.