Generally, running lower pressure will not make you go faster. Running lower pressure will give you better traction and might make the ride more comfortable. Running higher pressure will reduce your risk of pinch-flats if you are running regular tubed tires. When your friend said that running lower pressure made you roll faster off-road, he might have been trying to say that the lower pressure would give you more traction, therefore letting you hook up with the trail surface better.
-Moab