Avoiding all contact with the ex-husband

My mother and father have been divorced for more than 40 years. My mother has remarried and divorced about 8 years ago. My father has also remarried and is going for Hajj with my step mother. My sister and I disagree on my mother going to greet my father in and also attending family lunches. Is it permissible for my mother to be present for family lunches with my father in our presence and is she allowed to go see him and his wife off at the airport?