"Original song" - I would have thought that in the context of something billing itself as a "folk festival" this term should include traditional songs which had not been commercially recorded. For exmplke a song you found on a field recording, or in a book.

However, if the organisers perversely refused to accept that - which unfortunately is quite possible - than I suggest DebC could sing a song written by someone else, but which had not been commercially recorded. I don't mean pass it off as her own work - but "original" does not mean the same as "self-penned".

Have a hunt through The Mudcat Songbook (Original Songs and Music from the Folks at The Mudcat Cafe) (Note that "original"), and see if there's anything you like. And you're more than welcome to use any of mine.