Pilots - no, motorcyclists - yes. There are many spine injuries in collision between motorcyclists and more solid objects, mostly because at impact they are launched forward headfirst and often hit objects that way - with a hard helmet on the head there is nothing to dissipate impact energy, and this always leading to serious spine injuries - mostly in neck area, but that is not the rule. There is currently new helmet in development that will redirect the force at impact to help with those kinds of accidents.
While flying helicopters possibility of something hitting you from above - leading directly to neck injuries are very limited. Bigger issue are impacts of objects from the sides and front (birds, parts of aircraft, parts of landscape) The inertia of the helmet itself is mostly not sufficient to cause injuries, and when it does... well you wouldn't be better without it anyway.
To change your friend mind, you can bombard him with accidents reports where it say that helmet could save the pilot's life - like that one of TH-1F from New Zealand, that crashed because of hydraulics failure. Or before every flight, hit him in the head with map, kneeboard, flashlight... and ask him if it hurts
