Let me add to what I said earlier. The painted walls are likely to be damaged to some degree during the cleaning process, unless you can clean them with 409, warm water and minimal pressure. (not likely to happen, but maybe).
Make sure they know this and get it in writing before doing the job. The paint around the cooking area will be damaged already if it has had any degree of grease on it. If they are not planning to repaint the walls you may want to consider a weaker cleaner for the bulk of the work. I know this contradicts what I said earlier, but I should have asked some questions before answering.