Final answer:
The practitioner should respect the privacy and confidentiality of the frequent client in the colleague's waiting room.
Step-by-step explanation:
The most appropriate course of action for the practitioner to take is to respect the privacy and confidentiality of the frequent client. The practitioner should not approach or acknowledge the client in the colleague's waiting room, as it could potentially breach the client's trust and confidentiality. It is important for the practitioner to maintain professional boundaries and prioritize the client's privacy.