As a financial advisor, you have been working with a high-net-worth client for several years, helping them manage their investments and retirement planning. Recently, the client has expressed an interest in sustainable investing and has asked you to allocate a portion of their portfolio to environmentally-friendly investments. While you are aware that some sustainable funds may have lower historical returns compared to traditional funds, you also believe that sustainable investing aligns with the client's values.
In this context, what should your primary responsibility to the client be when managing their investment portfolio?