my friend lost a shulkerbox full of diamonds (64 in all slots) and claims he got all of them in 1 month. idk what kind of fortune he has. I kept denying that he said it was true now he unfriended me in discord. I did give him xray but that was around yesterday
TheMasterCaver says he got 91 díamonds from mining 10000 blocks, he doesn't use Fortune so that would be 3 stacks with Fortune III, 1/9 of a shulker box full, from 5000 blocks of mine tunnel.
Lets say 1/10, so you'd need 50000 blocks of tunnel for a shulker box full of diamonds.
Digging 500 block long tunnels would mean 100 tunnels or 3.5 a day, if I haven't messed up the math!, perfectly doable if you stick with it. (It's probably a bit less since the 10000 blocks probably includes ores mined from the sides of the tunnels.)
If you really want to know, inspect his mine and see if it's that big.
Proper branch mine with E5F3U3M pick (and high yield xp grinder to recharge) can yield that much in less than a week with no cheating.
You probably don't even need an XP farm; up to a third or more of all mined blocks can be ore (based on up to 1.7% being diamond ore (most players use a smaller spacing than optimal, such as 3 blocks, which has about half the yield) which is around 1/27 of all ores on layers 11-12; 1 diamond = 10 coal, 8 redstone, 6 iron, 1.2 gold, 0.8 lapis), averaging about 1.75 XP per ore and 0.6 XP per block, which restores 1.2 durability or 4.8 uses with Unbreaking III; only if you also wear full armor would it be an issue.
This is even more true of caving; I mined an average of 4375 blocks and gained 5300 XP per play session over the time I spent caving in a recent world (of course, I averaged only about 13 diamond ore per session and 3.4 per hour); even when spread over the maximum of 6 worn/held Mending items each one can gain an average of about 1767 durability, or over 7000 uses for an Unbreaking III tool (I have tested this before and had no issues keeping everything topped up, which isn't surprising given that I spend more overall to repair my gear in 1.6.4, especially back when I used a Fortune III pickaxe to mine everything while caving, at the cost of 0.9 XP per use and 5624 to restore full durability, compared to 4686 for 6 Mending items).