What do I do when a woman is taking advantage of my dad?

If you are concerned your dad is being taken advantage of financially, I would try to have a private conversation with him. If he is being abused emotionally, physically and financially, I would call social services to intervene.

And?

Your profile states only that your father lives at home.

Does he live alone?
Is he of sound mind?
In what way is this woman taking advantage of him, and what are you concerned it might lead to?