I'd probably try to set an example. Maybe ask for her help (rather than let her cook by herself) and then say something like "could you please wash the counter before we start, there could be some dust or cat hair on it" ... or "after you wash your hands, can you please wash and dice the tomato. I use a dab of soap on the tomato when I wash it. Thanks, it's a lot of fun having a good friend cooking beside me in my kitchen."