I don't know how things would work with a larger database such as reddit as I never worked on one. But I imagine if he had access to the database, he can just go in and change the comment on the database without writing any special program for it. I know reddit uses Postgre and that has a function that lets you go in and change field values.

Really, when you think about it, changing user comments would probably be a really easy undertaking for any forum administrator with access to the database.