Mine loves eating cardboard! There are so many boxes in the room for him to play with and chew on, but most are in tatters from his ripping chunks out of them. I give my bunny untreated and unfinished cardboard. It's just wood pulp, so it shouldn't cause any harm, especially if their gut is kept constantly moving by all the hay and fiber. I'm always a little worried that Luigi doesn't eat enough hay, but he'll often surprise me with an empty hay rack the next morning. Confining him in his cage overnight works wonders in terms of forcing him to eat all his hay. On top of that, he doesn't actually eat that much cardboard. If you keep an eye on your buns and make sure they don't eat an excessive amount, it should be fine.