I have a friend that fell over on his BMW GS on the highway while rounding a 25 mph curve. Although he was wearing a new top-of-the-line Shoei helmet, he apparently suffered a concussion.
Did you see the Harry Hurt Report on helmets back in the 1980's (IIRC)? A big study on motorcycle accidents and helmets. The best helmet for MOST motorcycle accidents (most happen at the bike riding at 25 mph or less) were the cheapest ones, such as the one WalMart used too sell back then for $15.00.
Faster speed accidents generally require a more expensive helmet.
His bottom line was "you tell me what type of accident you will have and I will tell you the best helmet".
But he also mentioned having the wrong helmet for your type of accident is always better than no helmet at all.
The problem with you friend's helmet could have been it was too expensive for the type of accident he had. Cheap helmets are softer and absorb more impact at slow speeds. The more expensive helmets are usually better at higher speeds, but worse at lower speed impacts.
If this has changed with new helmet technology since the 1980's, I do not know.
-Don- Auburn, CA