Final answer:
The nurse should discard the opened but unused transdermal fentanyl patch, document the client's refusal, and inform the healthcare provider to maintain medication safety and to reassess the client's pain management plan.
Step-by-step explanation:
When a nurse is preparing a client for a transdermal fentanyl patch replacement and the client declines it after the packet has been opened, the appropriate action for the nurse to take is to discard the patch as it cannot be reused due to medication safety and sterility concerns. Moreover, the nurse should document the client's refusal and inform the provider about the situation to ensure that the client's pain management needs are still being addressed, and to determine next steps in the treatment plan.