I don't think he will be able to help you on that since it's only admins who are able to view PMs as far as I know.
Admins and global moderators can do it.
Deleted posts are almost never removed from the database. A PM is removed from the database if the sender and all recipients delete it.
Full database backups are created daily, and all global moderators and admins can download the (encrypted) backups and implement their own rotation policies.