That's a bummer... And now she is gone so you really can't do much about it. I would probably take a chance to write a letter and ask for the ticket to be waived or reduced since it was for your Au Pair who has now left the country. I would not count on any local government employee to show much sympathy, but it won't hurt to ask.
FWIW, I did get a similar ticket in France with a friend's car, and they asked for the ticket to be waived and they agreed to...