It feels wrong to dissuade someone from building something

I understand this feeling, but it is important to push back against it, most importantly for the developer's own development. Not all ideas are good. I'd easily wager that most ideas are bad or at least not good, at least going from my own thought process. That is why we have places like HN, to share new ideas and get feedback. It is the best thing for the person with the idea (and also society) when their blatantly bad idea is responded to with generous, kind, informative, and direct criticism.