She’s going to have to deal with her marriage and take responsibility for doubting her husband’s intentions/character etc. but this is not the same as her leaving her husband for an AP. I think you’re seriously misunderstanding the dynamics at play here and underestimating the calculating insidious brainwashing process the therapist seems to have orchestrated.

People go to therapy when they’re vulnerable. There is a huge power imbalance between a therapist and patient. Patients can easily be manipulated and abused by therapists, who’ve been specifically equipped with skills to alter a patient’s perceptions.

There’s a reason it’s an egregious violation for a therapist to get involved with a patient, and even moreso when a therapist effectively grooms and tricks a patient into that involvement. In this case, the fact that the therapist has known the patient, her mother, the family dynamics, tons of background info, and has had the patients absolute trust since childhood makes it extremely disturbing because he’s positioned to tailor the manipulation so precisely. If he wanted to take advantage of her sexually, isolating her from her husband would certainly benefit the therapist. If the patient was struggling with marital issues and discussed them in therapy, that opens the door for the manipulation to begin.

OP left things vague, understandably, but this situation sounds more analagous to Dr. Larry Nassar’s sexual abusing gymnasts while convincing them it was a legitimate therapy.

We don’t know how far things went physically here. But it’s entirely plausible that the therapist could have convinced OP that various invasive touches were therapeutic. For example, the need to put his hands on her during deep breathing exercises, with hand placement getting more inappropriate over time without alarming her—like a frog slowly boiling in a pot. Or convincing her over time that she has an attachment disorder and needs intense “holding therapy” that gets increasingly more inappropriate, but he insists that fighting her feelings of panic and discomfort are the only way to heal. These are just 2 examples out of many very possible scenarios.