Mother involved in a haraam relationship

One of my friends has a very serious issue at home. His mother is having a relationship with a man for the last 10 years. She is still married to my friends father. The whole family knows about the mothers relationship with this man, even her husband knows. My friend tried talking to his family but no one wants to resolve this matter. He is very worried and his mother is still having a relationship with this man. He took all possible steps to resolve the issue but nothing happened. Kindly explain what he should do now? Can he break all relations with his mother and leave all of them?