Jealous delusions. What should i do?

My father is a mariner, currently on ship. My mother had been worryingly dull for a week and disclosed to me that my father is talking to some other woman in our family regularly. There is no truth to this and she is seriously being delusional. She came to this conclusion on basis that both have come online in whatsapp at the same time some times. She has had this doubt for about 1 and half years. I am cent percent sure that there is nothing going on like this. My father is a very busy man at sea. He speaks to mother for around 5 hours a day and rest of the time he is at work. This constant doubt is torturing him. I convinced my mother by debunking some of her theories and she was ok for 1 day. The very next day she moved onto next theory that they both are speaking on imo now. She understood when I convinced her about delusions. But she is very reluctant to seek help. She wont consult even online. She is always very dull. What should I do? Please suggest any medication if needed.