Since it's a PST, I'm guessing it's a POP or IMAP connection. Most of the times I've seen "lost" email, they've either been moved to a different folder or archived. First thing I would do is check if there are any additional .PST files, particularly archive.pst files, in the folder that contains his regular .PST file. Otherwise, I'd check each folder (including the drafts, sent items, etc).

We all these missing messages from approximately the same time frame?