First, I would not rush this, but be patient with the person. Sometimes a person puts up the psychological defenses of suppression, repression, and/or denial for a good reason. The person may need some time, for example, to get used to what happened before starting on the journey toward emotional healing. When the person is ready, you first can work with him or her to make that which is unconscious (repressed or denied, for example) now conscious. What helps is this: If the person has the safety net of forgiveness and knows that he or she can confront and eliminate that anger, then the person is less likely to fear the uncovering of that emotion.
Another technique is to make the person aware of his or her inner pain as a result of an injustice. If the person can look within courageously and see how much pain is in there, then he or she may be motivated to get rid of that pain. The first step is to examine the pain and label it. Are you in mourning only? Are you angry? Are you perhaps even furious? The diagnosis helps the person see the amount of forgiveness work necessary now to heal.