How do I keep a young girl away from my 87 year old father and prevent her from taking advantage of him?

While in a rehabilitation hospital, my father was getting taken care of by a young 29 something girl. , After he left there, she managed to get his address and phone number. She has been to the house and calls him everyday, this has been going on for seven months now. My mother has passed away so we moved my father to an assisted living facility 4 hours away from this girl. He is telling my sister and I that he loves her and is in love. She has got him convinced that he needs to go back to his house. There is money and property missing from the house and he keeps telling us that she is his friend. We are just so afraid that she is taking advantage of him but we don't know what to do. He cannot live in the house alone. Any advice or direction would be go greatly appreciated