Mermaid is a little cheaper than the others and I have heard only good things about them. A place down here called Weavers has good deals on them http://www.mmair.com/how_to_buy/deal...weavers_marine
I have done installs on a few boats and it is pretty easy if the boat is out of the water. And you have to be okay with drilling a couple of big holes in the hull Since the systems are precharged you just need to supply the peripherals like water and power. Tell him to take some time and realy think about where he wants to run the ducts-they need to be as big and as short as possible for best efficiency. One other thing-the intake portion must be perfectly sealed away from the bilge and the holding tank. Since a vacuum develops around the intake it will pick up even the slightest fume and endlessly circulate it in the cabin.