Free Traffic Legal Advice

Hello.  I would really like some advice on how to best deal with my daughters bailiff situation. She is quite disorganised and didn't change her address with DVLA for quite a while after moving house. She has an ADHD diagnosis which is recognised as a disability and recognised to cause disorganisation.  She has changed the address a few months ago though. She had a phone call from a bailiff telling her that she owns £500 for a PCN for a traffic offence from May last year so nearly year ago and has to pay now.  She has found the original PCN online and it is real.  The letters sent to her previous address were returned to sender so the council and bailiffs must know she never received them.  Bailiff said she is not allowed a payment plan but must pay now in full. This is within the Croydon borough and PCN issued by Croydon. What are her legal rights and what should we do to exercise them please?  Thank you.  LilliI