Its got to be something like a perception filter, they dont realise until they are told?
Whatever it is it scares them completely and utterly so i dont think its just looking at Ruby and seeing the truth, its more existential like they realise their existence is worthless because of her.
I think it could have just been them realising she was a ghost and her saying she was attached to Ruby in some way. Like imagine thinking that she's cursed in some way and the curse could transfer?