Well, we're clever, (says he, boasting !), but we're not miracle workers. The contravention does seem to be made out. She parked in a marked-out bay, so that gives her a duty to look for the sign for the bay to see what the conditions of use are. The sign is clear enough.
Whilst there may be a 'technical' appeal argument based on Harrow mismanagement of the enforcement process, these sort of things are usually only dealt with at the adjudicators, London Tribunals with the full PCN penalty in play. So if such an argument is presented here, she would have to forego the discount option.
Is there any mitigation you know about ?