Understanding Financial Leverage

Recently, friends mentioned that their periodic meeting with their investment advisor happened to coincide with their decision to purchase a new car. The investment advisor recommended that they take the dealer’s 0.9% financing because “our portfolio is doing a lot better than that.”
Which ignores the point that taking out the loan will leverage their portfolio. If levering up portfolio returns was such a good thing, why had they not discussed that as a strategy before? Investments exist that...