Do not work through your buyer’s subordinates.
Tell that person: “I’m sorry, but ethically I can discuss project progress and issues only with my client, who is then free to disseminate them to others. Ask John to give me a call or to let me know a good time to call him if this question is from him.”
Sometimes an assistant (real or virtual) will contact me because the person I’m coaching needs clarification, or lost something, or wants to schedule something. I always say, “Ask her to call or write me directly, things get too confusing working through a third party.”
There’s also the implication that your time is more important than mine, or that the buyer’s time is more important than your own. You don’t want to comply with a false belief.