Hmm... I've owned and shot a handful of Mosin Nagants and honestly without obvious serious malfunction of the gun itself I'm not sure HOW that would happen.
You say the injury is the actual casing itself penetrating THROUGH his hand? From one side of the hand to the other? Yet there is no obvious malfunction of the firearm itself and no clear damage to the gun? Even stranger yet - most Russian ammo is steel cased. This one looks like it might actually be brass...
Puzzling... befuddling... confounding... just plain confusing. I haven't got a clue but I hope your friend is alright.
All that aside I think someone out there might have something against your friend...