I hate the idea of "support" when it comes to entrepreneurship or business. The worst thing you can do to an entrepreneur is encourage them to pursue an idea that you personally think is wack.
You could be leading them on a path to wasting a large chunk of their life, not to mention money and resources doing something that's not helpful to anyone.
You don't have to be critical, just be honest. You're honest feedback could lead them to that right idea thats worth everything.