We are suspicious of manipulation of my parents by their caregiver. Any advice?

My father is 89 and mom is 96. They have a caregiver for my mom who has dementia. We hired her a year ago and over the past few months we notice manipulation from the caregiver. We want to fire her but my father refuses and he is paying her. My father has been scammed once and almost 2 other times. Any assistance will be appreciated.