If they are in assisted living, there is an "always available menu" (at least everywhere I've worked) in case they don't like whatever is on the regular menu for that meal. This usually includes basics like hot dogs, hamburgers, deli sandwiches, PB&J, grilled chicken breast, salads, soups, cottage cheese, fruit, yogurt, etc.

There is a good possibility something was being served she didn't like or couldn't have.

There is also the possibility she will only eat hot dogs, regardless of what her family wants. I have had many residents over the years who will straight up refuse to eat anything except the exact same meal, every meal, for years. (And I'm not talking about residents with dementia, I'm talking about residents who are mentally 100% fine.)

Just because someone is in assisted living doesn't mean their autonomy gets taken away. They have a right to refuse to eat things they don't like. They have the right to eat the same thing every day if they want. We can only suggest they eat other things, we cannot force them.

You also can't truly judge what a resident is eating throughout the day based on a single meal or especially a single picture. Where I've worked, she would have already been offered an appetizer prior to this in the form of a soup, side salad, fresh fruit cup, or something like a cucumber/tomato salad. She'd also be offered a dessert afterwards, and in this woman's case, a specifically sugar free one.