Unless he checked in the old files as a new rev, you can't
reverse the damage without redoing it all. Unless, of course, you
guys are backing up your vault nightly. You may be able to get the
correct files out of a backup.
Note about backups, when ever backing up the vault, tell your
IT dept they must do a FULL backup, not an incremental. The
incremental backups will only get the files added to the vault, not
the references etc. that can be scattered throughout the
vault.