This is the right answer but modified by an honest response to him when he inevitably asks your opinion on their prospects. "I see challenges..." [and list the

This is the right answer but modified by an honest response to him when he inevitably asks your opinion on their prospects. "I see challenges..." [and list them honestly, but don't overstate] is all you need to (and should) say. We've all looked at pre-successful businesses and thought they couldn't possibly work. Don't be so sure his will fail due to marketing requirements or any other shortcomings you perceive. Surprises surprise us all the time.