I agree with lv4dogs, get her checked if she still isn't feeling well. My dogs will sometimes eat cat poop and then later vomit. There's nothing more disgusting than kitty poop vomit.

Is she eating just the litter or is she eating the poop?

Maybe your parents can get new litter boxes with lids and flaps so the dog can't stick her head in. If the cat doesn't like the flaps, turn the litter boxes to face a wall just enough so the cat can slink between the space, but the dog can't. I've also seen this new kind of litter box with a top entry. It looks like a rubber maid container with a hole on the top of the lid so the cat can jump down into it.