The questions that help

Ask who the customer is and how many they have spoken to. Ask what it costs to find out whether this works, and how long that would take. Ask what would have to be true by a specific date for them to continue, and what they would do if it were not. These are the questions an investor asks, and they are useful precisely because they are not objections.

They also do something an argument cannot: they make the idea more real without you having to approve or refuse it. A young person who cannot yet answer them usually discovers that themselves in the course of trying, which is a far better teacher than being told, and leaves the relationship intact.