A customer who wants to show a tangible outcome faster is likely to have a high value for early and frequent delivery, which is a principle of agile and adaptive approaches. In such a situation, the project manager should review the customer's priorities regarding the desired requirements for the milestones, and discuss which requirements can be delivered faster with the project team. This is a way to apply the concept of value-driven delivery, which focuses on maximizing the value of the product and satisfying the customer's needs.
Informing the customer that moving the milestone has a very high impact on project cost, escalating the issue to senior management, or convincing the customer to keep the requirements as is are not effective ways to deal with the customer's request, as they do not address the customer's value or needs, and may create conflict or dissatisfaction. References: [PMI Agile Practice Guide], page 8; [PMBOKGuide 6th Edition], page 18.