As I have read answers and reflected on my own experiences I think a straight up "no" is not a good answer.
I agree that a therapist should not just come out and tell their client something like this. That is obviously unethical.
But I disagree that a therapist is "bad" or shouldn't be in the profession if they are struggling to control this or if it catches them off guard. They are human too. What is important is how they deal with it when they become aware of it.
If it is, in fact, effecting their work than obviously something needs to be done but the clients wellbeing also needs to be carefully factored in. This cannot happen with out talking openly with the client. Most of us are there because we have rejection and abandonment issues. We are scarred by people making decisions about us and then dropping us without communication. To try and transfer your client without any communication about it is the same. But even worse because this time it is a therapist and someone who is supposed to be trained to handle you…
In this situation I believe it is unethical to NOT tell the client. It would be unethical to withhold so much information from them that could help them understand that they do in fact have value as a person and maybe even more than they thought. It can actually be a profoundly empowering moment for the client when they learn that this is the case and that the therapist is wise enough and honorable enough to not act on it, and acknowledge that it is affecting them. It will help the client learn more than they would otherwise about themselves and what they can expect from other people and themselves. For some it may even teach them that they need not act on every attraction they feel and that respect for professions, boundaries, families etc. is more important than attraction.
Yes it will be painful, especially if they are attracted back, but it may be ignorant and naive, maybe even presumptuous, to think that the client hasn't already sensed that in some way. Without any discussion of it they are then left completely confused and broken not knowing or understanding at all what really happened. And a half truth response or withholding of important information is only going to feed their distrust in people.
I believe it needs to be an open discussion. Therapist, you don't need to admit everything but explaining that it is there and effecting your work with them and the reasons why it can not be entertained, I believe, are much more ethical then just passing them off with some bullshit explanation.
I hope that more therapists will discuss the reality of this and what I am trying to point out. In this situation, are you protecting the client or yourself? After you "break up" with them you will have people and training you can turn to but they will be left alone, again, and heart broken to try and sort it out and decide if they can trust themselves and/or another therapist. Please have the decency and ethics to let them know what really happened and don’t think you have openly discussed it if you have answered in the cover my liability, imply but neither confirm nor deny kind of way. That just adds to the confusion.