This is a great question and one that I’m sure could get lots of different responses based on the therapist answering. However, my short answer is that it is not the therapist who is in the wrong here as much as your stepmother. In this case it may have been ultimately unhelpful that the therapist offered a suggested diagnosis in the marital session, but not unethical. If she is a licensed mental health therapist it is within her scope to diagnose and if your father and stepmother are having trouble connecting and communicating, this diagnosis could help explain that. However, it would be best if he underwent a formal assessment in order to clarify the diagnosis and based on what you have shared, may be unnecessary.
However, the real problem is that your stepmother has used this information against your father. It may be that she is trying to justify her own behavior so that others would understand if the marriage doesn’t work. I’m sure it is difficult for you to watch all this transpire, but your father is the one in the marriage and has to be the one to make decisions about it. The best you can hope to do is share your concerns and observations and support whatever decisions he makes. Unconditional love can be difficult to express when we feel a loved one is being mistreated, but it is his marriage, not yours.
Imago Relationship Therapy, by the way, was created by Dr. Harville Hendrix and is based on his book, “Getting the Love You Want.” It is an emotionally focused form of therapy and often assumes a connection between the frustrations experienced in adult relationships and early childhood experiences.
I wish you and your father all the best,
Dr. Holly Counts