Final answer:
An engineer aware of a violation of professional conduct by a registered engineer should report this to the state's registration board to uphold the integrity of the engineering profession.
Step-by-step explanation:
When aware of a registered engineer who willfully violates a state's rule of professional conduct, it is important to maintain the integrity of the profession and ensure public safety by taking appropriate action. The ethical step to take would usually be to report the violation to the state's registration board. This ensures that the governing body tasked with enforcing professional standards is informed and can take necessary actions to uphold the integrity of the engineering profession. Reporting to the employer may also be appropriate if it relates to workplace conduct. However, reaching out to the affected parties could be considered, but this should be done carefully, and ideally after reporting to the registration board.