My 2 and 4 year old don't even know what McDonalds is...and I will keep it that way for as long as possible, so I know how the mom feels. I am not fanatical at all and we do eat out sometimes and I do bake cupcakes etc...but the thought of my child eating processed foods every day would stress me out too. Again, I think as long as she doesn't expect you to provide anything special for her child or do anything differently, it would come down to whether or not you are comfortable with the child coming with his own lunch. If not, then definitely tell her to find someone else, but if you are okay with that, then it may not be an issue. Just make sure you have a trial period so that if you are feeling unhappy with the arrangement, you can get out.