I wasn't there when it happened and don't know all the details. But I know that it was his habit to wear a helmet. Everytime I saw him on his bike or about to get on his bike he had his helmet on.
But it was a really nasty fall. As it was described to me, something went wrong with his front wheel while he was going really fast and he went over the handlebars. So maybe it wasn't enough to have a helmet.
I have heard about such accidents. My TREK, like most bikes has a rather easy release mechanism that allows quick removal of the front wheel. There is a similar release for the rear wheel, but of course there is a chain around that wheel. Newer bikes are built with more safeguards on the release mechanism for the front wheel.