A sexual assault? I would think it would be since there is no real consent, but there is no touching either.
That doesn't happen that way
I'm not a legal expert, but you can't even answer that question from an ethical point of view, just as it's given. It depends.
Was it agreed upon beforehand? Was it intentional on the part of the hypnotist or a side occurrence as part of the therapy? We need more details to give a proper answer.
I'm guessing, without touching, it would legally not be assault, but be very close to a felony sexual abuse charge - because the person is under the care of the hypnotist, leaving the orgasming patient as a vulnerable population.