I would need a bit more context to feel confident that my response matches your intent, but I’ll give it a shot & assume we’re discussing client/therapist relationship (i.e. in-therapy), without a 3rd party (e.g. spouse) involved.
The therapist SHOULD be multiple steps ahead of his client. He should have a strategy for intervening on what’s not working. And he should recognize that information that COULD be destabilizing, is likely to slow down the process.
In my view, good therapy is NOT about rational information. Quite the opposite in most cases. It’s about facilitating NEW experiences because the most important changes happen in the limbic system and not the rational/verbal center of the brain.
But in the case that the therapists leaks info before the client is sufficiently resourced, the client will almost always dissociate or repress the information rather than let themselves become destabilized. aka homeostasis preservation
With the assumption that the therapist is doing their best, and given the absence of free-will, I don’t ascribe much of an ethical component to the process. It is what it is.