Oh man, that's awful. Extra awful for the other rider involved.
I wish they had some details about his helmet. I for one would like to know what kind of helmet didn't protect him from a fatal head injury in a crash that didn't involve a motor vehicle or a fall from a great height. Since they say he was an experienced rider I would like to think that the helmet fit him well, was buckled and was within its useful lifetime ... but you never know.