Awfull, tragic story. That particular run is really mellow, but u can still pick up some real speed on it. Honestly, I was shocked by this. I was a medic In the Navy for 4 years & I never heard of severing an artery via whiplash!
As for a helmet "not" helping,.. I would suspect that in a situation like that, it could possibly contribute to some degree. The extra weight on your head!
Either way, terrible thing to happen to someone so young!
My sincere & heartfelt sympathies to her family & friends!
You're only Young Once,.. but you can be Immature FOREVER!