I have been bowhunting and competitive shooting for over 35 years and I have never seen what you describe. I have seen nocks break, and arrows shatter on impact, but never snap off like that. I think your hypothesis is as good as any, that the arrow was defective. I hope your friend didn't get too discouraged. If he has shot the other arrows without incident I don't believe he has anything to fear, nor is it likely something he did wrong. That's why it's called hunting and not killing.