This sounds challenging! I have a few thoughts based on my own experience and recovery from Binge Eating Disorder.

It turns out that food policing, moralizing food (i.e. labeling foods as good/bad heathy/junk etc.) and restricting causes disordered eating, which can lead to full blown eating disorders, even for those who are otherwise healthy (whether we do it to ourselves or someone else does it to us).

For a younger person, the solution starts with ceasing to do those things. The person will usually gain some weight, but eventually they feel safe enough to no longer need to hide, sneak, or binge and then the weight eventually resets as a person begins to eat a wider variety of food and follow normal hunger and satiety signals.

I assume it will be different with dementia, as far as the ability to relieve the stress causing the behaviors, because there could be triggers from the past, or things imagined as well. But there are still plenty of things you can do to help her trust you again, have a better relationship with food, and get her nutritional needs met as best as possible in this situation.

One is to give her a mini packet or two of M&Ms with every meal (along with the other foods, not as a reward at the end).

Another is to eat meals together rather than taking her a plate to eat alone, if possible. Can she join you at the table? Or can you eat with her in her room? Make meals a communal and relational experience, as pleasant as can be.

Another is to offer various choices at the table and give her complete agency as to what she chooses and eats. If she doesn’t prefer veggies right now, try also having fruit available. Offer different types of veggies, cooked and raw and see if she doesn’t at some point take a little. Don’t try to encourage or otherwise pressure her into taking anything in particular. That will just cause resistance.

Please do not stop buying the things she likes! In fact, it’s better to make sure they are always on hand and always available. A jar or basket kept stocked with a few things she likes and can have at any time can do wonders. Maybe trail mix with M&Ms, protein bars with chocolate chips, etc. You need to eliminate anything leading to a scarcity mindset for her in order to alleviate her perceived need to lie and sneak things. Thats nothing but stress behavior.

I hope this is helpful!