OK, I know this is going to sound kind of obvious, but here it goes...
make sure there is food available (at least in your home)for her that is similar nutritionaly to the kinds of food she would normally consume at home.
What perctentage (aproximate) of her regular foods were fruits, vegetables, grains and meats then try to acomadate her with our local selection of produce etc..
and when you eat out, make sure you go somewhere that offers a variety of meal choices. Even McDomalds sells salads
If she chooses to eat, exculsively, the burgers and other Americanized foods our country has to offer she will suffer the same fate as you on your trip to Brazil.