What a good cat that she used a litter box…it could have been worse, she could have used a house plant or your xmas tree 
I think it’s very funny when an animal goes through lots of drama to follow their code i.e. squeezing a big cat butt into a bunny’s domain and litterbox. I’ve had problems w/ the reverse, Jessica using the cat’s litterboxes. I wouldn’t care too much, but the cat’s have scoopable litter. Anytime I’ve tried pellets or carefresh they go on a pee revolt….so the evil scoopable is all I can do.
Is your cat fixed? If she/he is than I don’t think that the smell s/b too powerful for the rabbit. I would be more concerned about the cat’s feces. Definately set up kitty’s box, sounds like she wants it & is waaay jealous of the bunny’s box.
When I clean out the bunny litterboxes, 2 of my cats attempt to climb in to sit or lay down. I guess b/c they are just boxes, which my cats love, and bunny litter is overall much cleaner than the cats. I also think ewww when they try to nap in the bunny litterbox when I’m cleaning them out.