On Course Completion update external database

My scenario; A user in Moodle completes a course. I'd like to update an external database once this happens. My understanding is that the course_completed event is triggered every time the cron job is run. Which is the best approach to update an external database with some simple values e.g Username/ID, CourseID and maybe Date of Completion, of the completed courses? I'd rather not hack the completion/cron.php to do this, but will if I have to!