Final answer:
The nurse should document the finding and assess the client further.
Step-by-step explanation:
The appropriate action for the nurse to take is to document the finding. A reddened area on the client's sacral area can indicate a potential pressure ulcer or skin breakdown. Massage and applying lotion may aggravate the condition and should be avoided. Applying a warm compress without a proper assessment can also be harmful. The nurse should document the finding, assess the client further, and notify the healthcare team for appropriate intervention.