I'm a live-in nanny with a seperate flat. In my contract, it states that the family provide all my food and beverages. I eat with my charge Monday to Friday. I do the food shopping for my charge and myself so decide our weekly menu. I eat at 5pm with her because I am too lazy tired to cook for myself when I get in at 7pm. I am also supposed to go to the gym/pool three nights a week after work, can't just before I exercise and by the time I get home it's too late to eat.
I'd never take the piss though, my bosses are amazingly kind and generous to me so although we have a mainly organic diet I wouldn't buy finest fillet steaks or anything like that. If I am cooking for both of us, we'll share a chicken breast/salmon fillet, etc. although she's only 10 months at the moment so doesn't need much!
For the weekend I generally buy anything I want when I go shopping with my charge but pay for it separately, but I buy store cupboard ingredients (rice, salt, ketchup, mayo type things) from work kitty.
I would suggest to your friend that she asks her new nanny to add anything to the shopping list/delivery order that she wants but if she wants anything special (not sure what this would include except alcohol (for evenings of course!) and expensive meat and special treats like chocolate, sweets etc) she has to buy it herself. Then if she doesn't take the mickey then you friend can add her specials to the family shop.