While your husbands behaviour is not totally excusable (he is a grown man) I see severe manipulation on her part. She has gotten him to trust her and she is driving a wedge. Your husband needs a therapist who can make him independant, not dependant.

I would recommend seekign the advice of another therapist, who may know what avenues you could take to dealing with this house wreaker and other ways to approch your husband in a way that is trusting and allows him to rely on you instead of her.

Best of luck