Except the therapist works for the OP, not the other way around. If it were just OP’s friend who stood them up, then you’d have a point. But this is someone OP had an agreed-upon appointment with someone they are paying to treat them. And also keep in mind that many doctor’s offices will charge for a missed appointment if the patient didn’t show and made no attempt to communicate ahead of time.
Sure, there are probably understandable circumstances that have caused this, and the therapist will probably make it up to them. But that doesn’t invalidate OP’s feelings and expectations, especially in the moment.