I mean a pack of snacks specially for her or even a christmas present from grandparents. As I know, most of them don't like to work for a family with elderly. If she is a reasonable person, try to explain to her your difficulties and ask her to be considerate and repect to your parents.
Are you living with your father? If yes then you need to talk to your helper nicely. If not then try to give some snacks to her and tell her it's from grandpa. It works in my mum's case. She gives biscuits or candies irregularly to the helpers therefore they're happy to see her.