All I can suggest is to write a very apologetic letter along the lines of what you've written above.
You may check out the station where she bought the ticket to see if there are any posters that explain how the zones work, because I cannot see an explanation on the LUL website at www.thetube.com . It could be used in mitigation that there's no explanation of the zone system and that as a newcomer whe wasn't aware, had no intention of defrauding, had a ticket which she willingly surrendered to the inspectors at her destination station.



