Are you giving her kid friendly, but healthy food? Kids like Cheerios, Rice Cereal and milk. They like waffles and pancakes. They also like plain pasta with a little butter and maybe or maybe not some parmesan cheese. They like grapes and apples and mini carrots with ranch dressing. They like pizza and macaroni and cheese. If her parents did not get her used to the adult food that we in our 50s and 60s usually eat, she will probably take a while to get used to it. So, ask her what she eats at home and keep it very simple with maybe the above suggestions. I never met a small kid that does not like some sort of pasta boiled with a little salt, drained and mixed we a little butter. It is only for 2 weeks so give her what she is used to eating at home. Good luck grandparent.
It is important that she is staying hydrated. Let Mother Nature handle this - she will eat when she is hungry. Stop giving her dessert items. That will only serve to reinforce the pattern. How old is she? This behavior (i.e., shutting down from eating) is a way very young children show a depressive reaction to the loss experineced from being away from loved ones.