I discussed this with a friend who is a therapist. The office was located in a hospital, and it was located within several offices and large treatment rooms, so there was frequent traffic. When sxeeing an opposite gender client/patient, the therapist would ask a secretary to come to the door and open it at some point durinf the session hour. This was done all the time. when the sessions were sheduled at hours that the secretary was not there, the therapist npotified the security guard in the building to do a pop-in. The client/patient was notified of this.
This not only fulfilled the halacha requirement, as instructed by a posek, but it also provided a measure of safety that would be recognized by a secular court.