Hmm. I dont know what to make of it :O He must have went offline and duped a bunch of diamonds, and came back online, or he really dug for those diamonds... Or its a glitch. But thats a ton of diamonds to dig up if he did..
I have a world where I have legitimately dug over 10 stacks of diamonds. Logged a very large amount of hours on it. It just takes time, patience, and a lot of gravel filled lava pits.
Simple. Stagger mining. Go down to a few levels above bed rock, mine a 2x1 tunnel as far as you want, then move over 2 blocks, and do another 2x1.