Compare Radical Honesty Against Diplomatic Vagueness for a Friend's Failing Business Idea

Your friend wants your honest opinion on a business idea you privately think will not work, and both the blunt answer and the kind one carry a real cost. This weighs what each approach risks against the strength of the friendship, and hands you one sentence that sits somewhere between brutal and useless.

A close friend has asked what I think of a business idea I believe will not work, and I am torn between telling them plainly and giving a softer, more encouraging answer. In two short paragraphs, compare what each approach actually risks: what radical honesty costs the friendship and their confidence if I am wrong, against what diplomatic vagueness costs them if I am right and they find out later I held back. Then recommend which fits a close, long-standing friendship better than a newer one, and give me one sentence I could actually say that sits between the two extremes.