He is lucky it didn't break his back, the way the seat was mounted.

If you have a single layer seat, be it carbon OR aluminum and just have a spacer with a bolt through it, to space the seat off the hoop, you are taking your life in your hands. The seat will "fold" around that spacer and then you have a spear pointed at your back. You either need a big flat piece of plate to distribute the force to a larger area, or build a tubing frame to contain the seat. The latter is how I support the back of all carbon seats