If the voltage is dropping as the battery cools then that pretty much points even more to a shorted cell. I guess it would probably drop close to 5/6 of the normal voltage. Everything points to the battery and if your nephew says its not unheard of with AGM then pretty safe bet the battery is the only problem.
It doesn't happen that often with a regular battery where if the plates come loose they easily short so you would think that problem would be solved with the glass mat on the AGM but the they take advantage of that and put the plates closer together so who knows what else they didn't think through properly when designing the battery!
