“If you don’t have anything nice to say, don’t say anything.” I think it’s fine to be polite, but that sentence is incorrect. There are plenty of times where it’s quite appropriate to say something that isn’t very nice. If your friend is about to hire a contractor that you know from personal experience is a scam artist, then you should certainly tell your friend this!
But a key element is this: your not-very-nice comment must serve a kind purpose. Does it prevent harm? Does it help resolve a conflict? Then speak up. Something doesn’t have to be nice if it’s both true and helpful.
But those last bits are important – true and helpful. Without those, saying something that isn’t nice is just gossip.
