Hey, Lindsey. Sorry to hear about your friend. This is a good reminder to check all gear periodically. Thanks for that.
I've had harness straps and spreader bar hooks break at the peak of a boost, but I've always managed to "crash gently". If a strap breaks on one side, you're usually supported by the strap on the other side. You'll be lopsided in the air, but not totally unsupported. Also, if I had to find more than 4 feet of water for a boost, I'd never leave the surface. 4 feet is a lot of cushion.
I'm wondering how your friend ended up with a concussion and in the hospital. Did he hit his head on the board, or lose control of the kite and slam his head into the water? I don't want to take anything away from the purpose behind your post, but I'm curious exactly how the injury occurred.