E5 Ranger is right on the money,and I don't say that often 8^). "True friends stab you in the face", but for your own good. Whether telling them not to marry that harpie, or buy chinese multitools, not saying anything is a bollicks fail.
Some of my friends, I would just come out and say "You're a friggin' idiot!" and to others I would say "You may not have thought that through all the way..." You decide what will work best for each person or situation. The goal is to set them straight. Eventually, he should learn to deal with "friggin' idiot".