Final answer:
The nurse's priority action should be to lock the brakes on the bed to prevent further movement. The nurse should then assess the client for injury and inform the charge nurse about the incident.
Step-by-step explanation:
The priority action that the nurse should perform in this situation is to lock the brakes on the bed. By doing this, the nurse can prevent any further movement of the bed and ensure the safety of the client. After securing the bed, the nurse can then assess the client for injury, as falling to the ground can potentially cause harm. Finally, it would be advisable for the nurse to inform the charge nurse about the incident, as it is important to report any safety concerns in the healthcare setting.