There are a lot of laws and rules in the licensing process for therapists for this very thing, because it happens all the time. As stupid as it all sounds, he has a pretty strong case simply because its a well documented thing that therapists are specifically warned of and trained to not do.

Even if you don't agree with the "privileged and vulnerable position" idea that she abused her power over him by allowing it to happen, she was still under professional and legal obligation to deny and immediately transfer him to a different therapist.