If his thumb was in contact with the action lever then he had virtually no grasp on the rifle.To hold a gun that way feels unnatural. Betcha the action lever was stiff and wasn't totally closed at firing.

Was the action off face? Are the hulls defective? Have the other hulls been mic'd? I doubt the mild pressures of Doubles would affect the primer pocket like rifles--What did the primer look like?

Scary situation. I am a nimrod and I would defer to more knowledgible folks but I think I would take another rifle on that trip unless the Double gets cleaned and goes thru 50 factory loads with out problems(after a gunsmith feels its ok to shoot).