hmmm... I see where you're coming from. I'm not sure that indicates the person he's texting is her. It would be highly unusual for a therapist to admit an attraction to a patient, assuming they actually felt one. He may have interpreted her response to his admission as a reciprocal admission because that's what he wanted to hear. The fact that he said they'd be together "IF" they could, suggests they're not to me; and she set that boundary.

Tracing the number he is frequently texting is likely to tell you who it belongs to. Not necessarily, though, depending on who it is and how he knows them. It might look like he's texting cell to cell, but he could be using an app.