Hardware is incredibly expensive and time consuming to create from scratch in ways programmers usually don't appreciate. If the GP is asking those questions after thinking about the problem for years they do clearly need to be told they have no idea what the problem is.
In the hardware business we are constantly approached by people who have little more than a vague idea and a hobby-money budget. I've seen what happens when you encourage them, and it usually involves fucking up their retirement if you let them get too far.
Much more humane to see it for what it is and shoot the idea down as fast as possible, or at least help them understand they probably need VC level funding to get anywhere.
If someone has been simmering on an idea for years and can't even formulate a clear explanation of a problem he wants to solve (so that people can explain what's required past "it's really expensive, don't do it") it's clearly not sinking in and there's no product there, just a guy who wants to play with technology and waste manufacturer's time while doing it.