First, it would be best to have him think as carefully and as rationally as possible to sort out what is true and what is false regarding the mother’s actions. He needs to take a courageous view of the truth of the mother’s actual injustice. Once this occurs, he should be able to see the exact injustices in which the mother engaged. Your roommate then can pick out one incident and forgive his mother for that one. Then he can move to another incident. Little by little, he may forgive so that his resentment lessens and he can consider approaching his mother with a deeper sense of her inherent worth.
For additional information, see What Is Forgiveness?