If you use the standard BMR equation for a woman..
BMR=}[(4.35*mass) + (4.7*height) - (4.7*age)] * activity}
You can come up with the minimum caloric intake for a day..
Since you didn't include your grandmothers size, age, and mass, an assumption of a woman who is 80years old, 5ft 2in, and weighs 90lbs would net you a BMR of 1057.9kcals a day (calories) If you say that she was pretty much sedentary, the activity part of the equation would be about 1.1 to 1.2, yielding you a max of 1269.5cals a day. Neglecting all dietary needs that are not strictly calories (i.e., vitamins, proteins, and carbohydrates etc') you can come up with her daily caloric need. For example...
If she drank 4cups of tea a day@ 5cal each, consumed a total of 8oz of milk in a day at 150cal's, drank a nutritional supplement (I used Ensure for calorie content) at 355cal's, you would be left with about 525cal's, which is not nearly enough. However, the sugar that she would have consumed would have plenty of extra cal's. If you say each 15g of candy has 75cal's, (somewhat conservative I think) and she only needs 750 cal's more, that is only 150g a day, or three to four candy bars...
These numbers are only for example of course, as there are other variables. But something to consider is that your physical activity is only about 20-25% of you caloric usage, just living takes about 70% and the digestion of food takes up some more, as does producing heat. In the end, someone who ate less would have less energy expended on digesting it. Not to mention that the older you get, the less food you need....