If he needed an old email, he'd just go search in the deleted items.
I have asked users before, "would you keep your lunch in the trash can?" Then I spent some time showing them how to use mailbox folders or archives.
I never really liked the archives because they were always stored in the local Application Data (or %localappdata%) directory which is not subject to roaming profiles or folder redirection. If the user moved to a new computer the archive PST would have to be moved manually, or worse if the computer tanked it was lost. Storing the PST in "My Documents" is not much better because Outlook has the habit of continuing to run after its GUI is closed, thus holding open PSTs which would wreak havoc with roaming profiles in particular since users have the habit of logging off or shutting down without closing programs.
It rather amuses me how Outlook now likes to use the "Outlook files" folder in "My Documents" and Microsoft encourages the use of redirected folders, considering Microsoft also warns against using PST files over the network. (Unfortunately, the blog post link in that article is no longer a direct link but can be found in plenty of other resources. GFI has a really nice write-up on this.)