When you talk about your T 'kissing you back' do you mean she returned the peck, or do you mean a more lingering type of kiss? I think there is a distinction that needs to be made here.

Because... if she simply returned the peck, simultaneously, then I would tend to think that she was caught out in the moment by having her face grabbed, and pulled towards you, and then being given a kiss by you. If that is the case then i think people in this thread are judging her too harshly. Of course a T should *never* kiss a client*. But, if in the space of a microsecond you are kissed and the socially acceptable thing to do if one is 'given a peck' is to 'peck back', then I would be very willing to forgive her and not consider it an intentional boundary crossing on her part.
I would be very willing to give her the benefit of the doubt about her being able to maintain appropriate therapeutic boundaries at other times. unless of course something else happened. Then I would reevaluate.