Re: I don't get it.
"Can registry changes make any difference to the loss of data in a PST file?"
Hmm. Mostly I suspect that the team leader believed that messing about in the registry performed some chicken bone ritual. It had worked for him before so try it again... There are so many implementations of Outlook and so many app versions that there might be a bug which prevents correct reading of a PST file.
If running an enterprise Outlook/Exchange environment, wiping out HKCU settings and anything in %AppData% for Outlook would be relatively harmless. The user would lose a few preference settings but the mailbox would be auto configured to work again.
Wiping out HKCU etc in a non-enterprise environment would probably screw up Outlook and require tech intervention.
Wiping out HKLM entries for Outlook is bad advice. You cannot be sure that Outlook will apply sensible defaults when it finds them missing.