John's Responsibilities to Clients

John is a portfolio manager at a large investment firm. He has been managing a substantial fund that primarily invests in technology companies. Recently, he received a request from a corporate client who wants him to invest a significant portion of their newly allocated funds into a specific tech startup that the client has a personal connection with.

Although John knows that the startup has been struggling to meet its development milestones and has significant financial risks, the client insists on this investment due to their personal ties. John is concerned that proceeding with this investment could violate his fiduciary duty to act in the best interests of all clients.

In this scenario, what should John do to uphold his responsibilities to all clients?