Explanation
According to the PMI Code of Ethics and Professional Conduct, project managers have a responsibility to respect the confidentiality and privacy of their stakeholders1. If they inadvertently disclose confidential information, they should promptly inform the affected parties and take corrective actions2. In this scenario, the project manager should inform the client and their supervisor about the mistake and apologize for the breach of confidentiality. They should also ask the other manager to delete the email and confirm that they have done so. This would demonstrate honesty, respect, and professionalism. References: 1: PMI Code of Ethics and Professional Conduct, Section 2.2.4 2: PMI Code of Ethics and Professional Conduct, Section 5.3.2