Final answer:
The nurse's next action should be to immediately inform the physician.
Step-by-step explanation:
The nurse's next action, if the client's white blood cell (WBC) count is 25,000/mm3 on her second postpartum day, should be to immediately inform the physician. A WBC count of 25,000/mm3 is significantly elevated and may indicate an infection or another underlying condition. The physician should be notified so they can further evaluate the client and determine the appropriate course of action.