First, stop saying it can't be done. Some will try to do it just to prove you wrong. Because, quite simply, it
can be done. People do it all the time. It's called a brevet, or an audax, or a randonnée, or ultra cycling.
Second, since we're talking a brevet-length course, we need to be talking brevet training. Google on the topic and help them learn what it takes. There's more to it than big brass balls. In a nutshell,
A brevet should be undertaken only after a course of training to prepare for the ride.
Training should consist of several shorter rides during the week followed by a longer ride - of an increasing distance - on the weekend. Once a weekend ride of between 120 - 150 kms can be done with comfort, a cyclist is ready to take on the challenge of riding a first brevet. —
http://www.randonneursontario.ca/tech/tr200.html
Big brass balls can probably get any of them through the first 150 km--even on a POS bike. It's the second 150 km and the time constraint that needs the training. Get them to acknowledge that, and you'll gain some headway.