I am not sure why you need a tamper proof feeder? If she has plenty of visitors, they can feed the dog, can't they? So then you'd just need to prevent her having access to more dog food (although of course that won't stop her giving it human food).
My mother over-fed the cat, so her carers fed the cat and then hid the boxes of cat food so she couldn't keep putting out more food.
In terms of a tamper proof feeder, I doubt you'll find anything suitable. People with dementia are fond of dismantling gadgets.