If your friend is persistent about keeping her bun outside – sometimes parents etc can be very reluctant to let a bunny live indoors – then there’s always the risk of parasites attacking her bun. She has to treat the bun when the mites manifest themselves + take extra good care to give the rabbit healthy, high-quality food. Good healthy food helps keep the immune-system in good working order. Mites and other parasites benefit from and take advantage of lowered immune-defenses in animals and humans.
The hutch should at least be painted, on the inside as well as on the outside. (But not when the bun is in it of course, paint-fumes are toxic.) Painted wood is much easier to clean and doesn’t provide as many “hiding-places” for mites, bacteria and other parasites and microbes you’d want to avoid. The paint should be for carpentry outdoors and high gloss and in a light colour because light colour makes it easier to spot insects and dirt and sticky poop and other unwanted stuff. The hutch should also stand on legs or sth so it isn’t directly on the ground.