If the psychologist believes that his patient’s symptoms, depression, conflicts, anger, misbehavior, poor functioning, etc., are a result of his parent’s inappropriate, destructive, disturbed behavior and relationship towards him, can he make the patient aware of this and in certain situations, encourage him to express his resentment and anger toward the offending parent in a controlled and appropriate manner, since the goals of these interventions are for the purpose of enabling the patient to rid himself of his symptoms and conflicts and function better in life (“to’elet”) and not to degrade or embarrass the parent. Also, these interventions will prevent the patient from continuing transgressing the prohibition of “hating a person in his HEART”.
If the only way in which the patient’s symptoms can be relieved is by revealing the root of the problem, then certainly this should be done, and this will not be considered lashon hara.
As you note, if the problem lies with the parents, then doubtless the relationship is already bad, and revealing the problem will hopefully enable the patient to come to terms with the situation, so that in the long term even the relationship with his parents will improve.
Good luck and best wishes.