Because we currently live in a culture in which constructive criticism is seen as negative criticism, there are now certainly guidelines to follow when dealing with someone's creative work.
1) Never offer any unsolicited criticism. Ever.
2) If they ask you for criticism, ask them first what kind of criticism they are looking for.
(NOTE: most people these days want nothing but praise. Anything less than OH MIGAWD THAT IS THE MOST FANTASTIC CREATION EVER IN THE HISTORY OF ALL HUMAN ART will be viewed as negative criticism)
3) If they ask for HONEST criticism, speak ONLY of the positive aspects of the art. No matter how banal.
4) Then, when you are done with that excruciating exercise, go have yourself a drink and read something by Oscar Wilde or Dorothy Parker, and imagine yourself as their contemporary, at a time when honest criticism was viewed as true and honest and pure, because tearing apart someone else's art showed that at least you CARED enough about it to shred it.