The real question to answer is, does his idea suck? I think a good way to answer this question in general is to ask him for specific examples of how he envisions users using his product and why. I.e. who the user specifically is and what he/she is thinking before and after adopting the product.

The great thing about this question is that it's solely a test of the idea. If his idea (whatever iteration it's in) is any good, some number of people will want it. It's a theoretical exercise that excludes the later question of whether he will execute well.