Final answer:
The nurse should immediately notify the healthcare provider when rebound tenderness and involuntary abdominal rigidity are observed, as they can indicate serious abdominal conditions requiring urgent care.
Step-by-step explanation:
After observing the presence of rebound tenderness and the onset of involuntary rigidity of a client's abdomen, the appropriate action for the nurse to take is to notify the healthcare provider (HCP) of the findings. These symptoms are indicative of peritonitis or another acute abdominal condition that may require urgent medical intervention. Placing a warm moist pack on the client's abdomen or administering pain medication without a diagnosis might mask symptoms that are crucial for a definitive diagnosis. The semi-Fowler's position may help with comfort, but it does not address the underlying issue that requires medical attention.