What to do with Dad as a “facebook stalker?"

Dad is 86 years old and is continuously browsing friends and family posts and leaving annoying or humiliating comments. While not necessarily been threatening, doing this constantly seems to me as an obsessive or dementia behavior. It is causing my mother great emotional distress. When we visit him all he talks about is what he has seen or read in facebook, even from people I had never met. He even gets angry when the computer doesn’t work. I let him know some comments are hurting his loved ones, and it’s becoming his building an anti-social network, but he doesn’t care. I closed my own account and other relatives have blocked him because of his constant monitoring. I told my other sibling which lives in other state, but he is not so supportive. And believe me, dad is never interested in any leisure activity.