I have read plenty of stories like this, almost everyone of them was due to a weakened arrow shaft. I always flex mine and listen for clicks or sounds like splintering carbon. I also run my hands up and down and visually check for damage. I started doing all this after a story exactly like this one. The most common problem with this type of injury seems to be nerve damage in the area, atleast that is what it seems like after reading quite a few stories. Hope your buddy makes a full recovery.

Shane