I don't see why doing a quick head check would be his fault and causing the crash. He may have been following too close,
but aren't we all guilty of that to some extent.
Just because we all do it doesn't make it okay. If he was following so close that during a quick head check a car stopping (no matter how fast) caused him to run into said car, then he was *way* too close. While I hope the rider is ok, I would blame the rider for the accident in this case.