[codezero]

Tell them only what is good about their idea.

This isn't meant to be a trick either. You should think really hard about what is good, and what works with their idea, and let them know what it is and why. They can use that to iterate and make something better. Many peoples' friends bad ideas end up solid products, but not because they were told what was shit, because they were told what was good and they optimized for it.