How awful! You say your dad is getting frailer and has memory problems (has he been to the Memory Clinic)? Depending upon his needs, he may be a vulnerable adult, which is a whole new ball-game, but old age alone doesn’t make a person a vulnerable adult. Is he able to meet all of his needs independently (cooking, medication, self-care)?
She appears to not be gaining anything from having your dad there, so why would she want him to stay, other than having someone to be unkind to (horrid).
You could try a different approach….. I’d be tempted to go see the woman to call her out on her appalling behaviour and ask her to give your dad notice to quit.
