How do I revert the changes made by a specific user?
If you don't have revisions you can't rollback the changes unless you have a database backups that you can use to restore and old copy of your site.

If you don't have any strategy to save your database you can consider use the Backup and Migrate module, for Drupal 6 the last release is the 6.x-2.8


  Back up and restore your Drupal MySQL database, code, and files or
  migrate a site between environments. Backup and Migrate supports gzip,
  bzip and zip compression as well as automatic scheduled backups.