How to stop a potentially harmful relationship

My grandmother is 82 and has outlived 3 husbands. I have been caring for her since 2013 when she lost her last husband. We are very close and live on the same property just in separate homes. Last year she lost her daughter to cancer. At that time a man took the opportunity to play on her need of comfort and began visiting daily. This man dated my grandmother daughter in the 70’s and dumped her to marry someone else. The woman he married was my aunts first cousin. Now this man was a part of the family bc of marriage. When my grandmother lost her first husband (my grandfather) she  became close to my great uncle who had just lost his wife (my grandfather sister). This uncle married my grandmother. This made the man who dumped my aunt my grandmother son in law now through marriage. When my grandmother second husband died his kids took everything they could. This man was part of all the pain bc he was married to the oldest daughter. So here we are 15 years later and he has convinced my grandmother she’s in love with him. She will not listen to reason. She thinks we are just trying to take away her friend. This man is known to be a leech. He lives off people until they don’t have anything left. She refuses to listen to me and continues to see him. Her memory is slipping and she has good and bad days. I’m very worried that he’s going to take everything from her and she doesn’t have much. Any advice?