My mother is 94 and a good eater. She is not over-weight. I enjoy cooking and prepare healthy meals and snacks every day - no processed or junk foods. Lately, she is hungry every 2 hours or so and asks for more to eat. She is disabled and gets little physical activity. I'm perplexed as to why she is hungry so often.
I have spent some time learning about nutrition and it is amazing how many empty calories there are - an example I heard about the other day was that people who eat instant oatmeal for it's healthy properties get hungry after a few hours because the instant isn't as nutritious as the cooked from scratch kind. Maybe you might consider asking her md for a nutritionist consult and taking in a food diary when you go. Especially if she has just started it may be that, it may be medication - anything is possible.
A person can literally forget to be hungry, or she may forget eating lunch 10 minutes ago and may want to have the meal again. For this reason, I think, It's very important to set up a regular meal time and stick to it, and all must be calorically controlled. If she wants to eat right after a meal, put out a plate of small snacks such as cheese cubes or a sliced banana.