According to the Professional in Business Analysis (PMI-PBA) Examination Content Outline1, one of the tasks of a business analyst is to "communicate and manage solution scope boundaries and approval processes to stakeholders" (Task 5.2). This implies that any changes to the solution scope should be approved by the relevant stakeholders and follow a predefined process. The PMBOK Guide2 also states that "the project manager, project team, and other stakeholders are responsible for determining how the project scope will be managed, how scope changes will be integrated into the project, and who will approve the scope changes" (Section 5.6.1). Therefore, the project manager should advise the team not to engage in modifications unless it follows the proper change approval process, as this would ensure that the scope is controlled and aligned with the project management plan. References:
1: Professional in Business Analysis (PMI-PBA) Examination Content Outline
2: A Guide to the Project Management Body of Knowledge (PMBOK Guide) - Sixth Edition