Telling a friend their haircut looks great is kind. Telling them a bad plan looks great isn’t.

There is a difference between a harmless kindness and agreement that costs somebody something. A haircut cannot be undone by honesty and nobody is harmed by the compliment. A plan can still be changed, which means your real opinion is useful right up until the moment it is too late to act on. Saying nothing in that window is comfortable for you and expensive for them.

Ask yourself whether the thing can still be altered. If it can, say the doubt once, clearly, and then let them do what they want. You are not obliged to repeat it, and they are allowed to go ahead.