Fair enough this could be an issue where the parents chose to coddle her instead of teach her boundaries. I’d phrase it something like this “Look I’m just gonna be straight with you and tell you that you can’t practice your kink in shared spaces and you definitely can’t do it without cleaning up after yourself because everything stinks now. Bare minimum you owe me a new couch because of this because cleaning will not cut it in this case.”
This isn’t one of those things where you can take the soft approach you have to be direct and hit all your points clearly. She might ask for further explanation so when she does try to avoid using “because it’s disgusting” as an answer. Not because it’s not (it is), but because she might continue to ask why. It’s better to explain the specifics of why it’s disgusting (it’s unsanitary, it ruins your furniture, it smells awful, etc) to drive the point home that she can’t do that.