That is very tough. My Mother has never been a big eater, and now has Dyspahgia (along with Dementia), which is a swallowing disorder. She totally stopped eating while my Dad was so ill, and was down to 72 pounds. The caregivers have been making her shakes that have protein powder, ensure, ice cream, and peaches -- but make it thin enough for her to get down. We also add protein powder to puddings, or whatever she can get down. Also switched to baby food as it's more pureed. She likes the sweet potatoes, go figure. And we've also added protein powder to the cut and bake chocolate chip cookies. It's funny because when she used to cook -- it was always a meat, starch, and vegetable; and now I can't get her to eat any of it!
At this point, find something she does like to eat. Is it chocolate? Sweets? Salty? And then go from there. Throughout the process of my Mom's illness I've learned that many elderly lose their "taste" so sweet or salty things is what they prefer. I cringe because I think she is not eating properly, but at this point they just need to eat something.