While I will defer to the metallurgical experts on this site, I'd have a hard time accepting that the factory used a process to attach the ribs which would have so overheated the barrel steel as to weaken it. Seems to me to be too likely to cause problems and would therefore be rejected by a prudent factory.
IMHO, the most likely working theory seems to me to be (a) an obstruction in the lower barrel causing a burst, (b) a near-simultaneous discharge of the upper barrel, either as a result of (I) a flaw in the trigger or (II) the shooter involuntarily pulling the trigger (again if a ST gun, or the 2nd trigger if a DT gun) in the middle of all the other hell breaking loose, (c) either (I) a part of the lower barrel slamming into the upper as the lower disassembled itself, causing enough of a deformation (possibly elastic rather than plastic) in the upper to act as an obstruction such that the ring bulge was created when shot #2 came along, or (II) a second, smaller obstruction waiting in the upper for that shot #2 to come down the tube, creating the ring bulge.
(c)(I) would seem more likely if there were marks of some sort on the upper barrel's exterior, the existence of which I can't suss out one way or the other from the photos.
(c)(II) would require something extremely unlikely to have happened, i.e., a bad batch of base wads/shells such that multiple rounds in the same box left obstructions behind. It's (relatively) rare enough to have one bad shell, but to have 2 in close proximity has to be really rare. I could see this happening if the shells were reloads, but if they were factory loads then all the red lights should be flashing about this over at Federal.
Perhaps your friend has either the box from the shells, the receipt for the shells or bought them with his card. In that event, the manufacturer (in cooperation with the retailer) might be able to track the shells' manufacturing lot and pull them.
