The heat is from the bulb and the air around the bulb, A cool tube will allow you to get the light close or even a heat shield will work. You still need a blower to move the air through the hood or cool tube and out the other side.

For the record I can get 1000w hps as close as 12" and the plants love it. Yes I do have AC in that room but there is no cool tube. The lights come on in a few and I will show you how close it is.

Your friends had other problems; there are a few nutrient problems that have the exact same signs.