I've found one of the most effective ways to keep mites and lice off my chickens is to make sure they can dust bathe regularly. I have an old cat litter box that I keep filled with sand and wood ash. Wood ash seems to be particularly effective for this. I keep it in a covered area of the run so it stays dry and they can dust bathe in it whenever they want. You can also add poultry dust to the mix or DE if you want. But I agree with other posters that if you have an active infestation you must give the chickens a good dusting with sevin, pyretrhin, permethren, poultry dust or whatever to clear the infestation first. And you have to repeat it weekly a few times to make sure you get the eggs. You also may want to spray or dust the inside of the coop to prevent re-infestation. After that, make sure they can dust bathe and that should keep the critters at bay. Also, don't worry about your cousin picking up lice from the chickens. Chicken lice and mites are species specific. That means they can't live on humans. If you have a bad infestation you might see a few on you, but they won't stay and multiply on people.