“Stop giving her food. Stop trying these compromises like healthy snacks only or just one meal a day (!). Stop entirely, both because you shouldn’t have to be feeding her at all and because I think it’s going to be easier if you have one consistent message.”
The suggestion is guaranteed to work. Under no circumstance give your food to this person. It’s definitely unreasonable of the boss to be asking at all and there’s not much control we can have over the boss’s behavior. We have full control over our behavior.
Don’t give food. State that you’ve talked about it. I’d even say privately that it’s really unusual behavior and tell this manager that I hope everything is okay.
At that point, you can accept that this manager is strange. That may mean exiting this job sooner than originally planned.