These are all valid concerns and hopefully if the T sees this happening she will get control of it on her end. But if he does "fall" for her it could be difficult for you guys. Have you seen changes in him to make you think he's falling for this therapist? Has you husband cheated on you before? This is way more complicated than he just likes the therapist a lot. Maybe this is something that you could talk to him about and discuss transference with him.
Let him know how you feel and see what happens.