This is incredibly uncommon.

While horrifying for your friend, it's very low on the list of the risks that come with bikes.

I don’t know how common it is, but why would you take that risk on the road for a couple kg of weight savings? If you’re racing competitively that’s one thing, but is it worth it on the commute?

It might be less uncommon than you think. https://www.outsideonline.com/outdoor-gear/bikes-and-biking/...

Was the frame damaged before? I have ridden 4k miles on my carbon road bike and I've never felt like it was going to fall apart.

I ride road/street almost exclusively, and the Bay Area roads don't do my bike any favors

Carbon fiber frames can be challenging to see the cracks and damages to it before it spectacularly fails. There's a good chance you won't know it is about to break until it shatters.

Nope. As far as he knew it was in complete working order with no suggestion it was about to fail catastrophically until he woke up on the floor.

Any material under stress gets fatigued. The problem with carbon fiber specifically is that there are practically no visible signs of it reaching the threshold.

Some materials (steel and titanium being the most common) have infinite fatigue limits. They can be designed to not get fatigued.