Well, he is in his fifties, male, divorced, and no friends that you know of. What does he do for a living? Is it a job that brings him into contact with a lot of people? Does he have many colleagues? Does he work in a team? Is he a long distance lorry driver? Barman? Salesman? You dont have to answer that, just think if he has an occupation which is largely solitary or not.
I read somewhere that most people built their most important friendships in primary school, and have their closest friends where they grew up. Does he live in his home town? Or has he moved?
Otherwise people have fleeting friends through work and groups, and these often dont last when you change your job, join another group etc.
Most of your friends are through your child.
He is divorced. He neved did the "mother and baby circuit", so did not build friends there. He is divorced, and his wife kept the friends.
If he lives in his home town, has stayed in the same job the last decades, and have no friends, I might be a little concerned.
But if he has moved, gotten divorced, changed jobs, etc, it is quite normal not to have built new friendships. It takes effort, and is quite difficult outside the confines of education and motherhood.
Friends come and go! There is nothing weird about that in itself.