According to the best practices in project management, when facing significant changes that impact project timelines, the project manager should first assess the situation with the project stakeholders. This involves analyzing the new circumstances, evaluating the potential impact on the project delivery, and exploring alternative solutions or adjustments to the project plan. Once a clear assessment is made, the project manager should then communicate the impact to the customer, providing them with the most accurate and updated information. This approach ensures that the customer is informed of the situation based on a thorough analysis and collaborative effort, rather than a hasty, unilateral decision by the project manager.