How would you feel about a therapist developing a sexual relationship with one of YOUR kids?
Take a stand both for your marriage and for this girl, who is mentally unable to do so for herself at this time.
Gently talk to your wife and tell her that this unhealthy R has to go. If it doesn't, you will be obligated to go to the authorities.
Don't be afraid of the consequences; what you have now with her is not necessarily a healthy marriage anyway. And it's NOT going to change unless one of the three parties forces it: either you, your wife, or the girl.
