If asked, I give constructive criticism, which they can act on or not at their option.
If I believe something in their idea may lead to problems, I tell them about the consequences I see. If they disagree, that's fine. I've been wrong before. Judging or evaluating isn't necessary or helpful.
Sticking with constructive criticism and describing consequences one sees helps in many areas beyond start-up ideas.
I think it (assumption that friends are afraid) doesn't really apply to the majority of HN audience. We give and read criticism (constructive or not) every day here, it's the second nature for us.
However, we are a small group of people. The OP's post strongly applies to a more casual audience, who won't read this post.
If asked, I give constructive criticism, which they can act on or not at their option.
If I believe something in their idea may lead to problems, I tell them about the consequences I see. If they disagree, that's fine. I've been wrong before. Judging or evaluating isn't necessary or helpful.
Sticking with constructive criticism and describing consequences one sees helps in many areas beyond start-up ideas.