I don't get it.
His boss was an arse, yes.
Can registry changes make any difference to the loss of data in a PST file?
No. No it cannot. so other than the TL being a tosser, this just doesn't stack up. The data was either present before the registry change, or it was not. The registry changes had no effect on this at all.
Then we move on to this this tool he's familiar with, but totally didn't think to use until phoning base and asking for advice. He then remembered and "saved the day". What exactly did he try other than recreating the mail profile and (presumably) running the baked in scan/fix PST jobby in office?
He ran this on a backup he'd taken, so basically the PST file as it was when he arrived (which suggests the data was not lost at this stage) just a corrupt PST, that';s probably fucking huge.
He was given poor advice, by a dickhead, but the dickhead didn't cause or make the issue worse.
I also suspect he walked off site leaving the same person with the same mega-gigabyte PST file that will corrupt again in short order.