Helmets won't help when you are tumbling. They help with direct impacts reducing blunt force to one spot of the head and also reduce the bounce that occurs when you smack your head.
I have a feeling she injured her neck in the tumble.
True, I have not seen enough details. I hope she caught her heal side edge, slamed her head and tumbled after.
It really does not matter, it is a tragedy anyway that it happened. As a father with young kids, it would just make me feel better if there was some way I could point to some small thing and say "if she was wearing a helmet...."