I'll go along with the barrel obstruction theory. Probably a squib load fired before left a bullet in the barrel just passed the forcing cone.
Yep, one lucky guy for sure.
If he hasn't already, he needs to contact S&W, send them the gun and let them examine it.
I also have some questions.
What ammo was used?
How old was the gun?
How many rounds had the gun fired before it blew?
While it could be a metallurgical issue, I'll stick with the obstruction theory.