You really must respond to the Notice to Owner or they'll serve a Charge Certificate which adds 50% to the amount due.
As Stamfordman says, submit some reps based on your narrative, and emphasise the pregnancy which probably caused her to miss paying and also miss responding to the PCN. The best outcome would be they re-offer the discount. If they don't then you can register an appeal at London Tribunals and hope the adjudicator recognises pregnancy can affect the life of the mother-to-be in more ways than one.