As far as criminal law goes, it's not an offence, per se, to ignore directional signs on a supermarket car park but you can still be prosecuted for offences such as 'driving without due care and attention' on any land to which the public has free access (including private land such as supermarket car parks).
However, it seems that this is really a civil dispute. As Dilligaf states, there was a duty on the driver reversing to ensure that there were no pedestrians walking behind her car, so quite clearly, she has failed to exercise due care.
Equally, however, there's a duty on all drivers to exercise reasonable 'hazard perception' skills. That means that, when driving around a car park, drivers should always be watching for other drivers returning to their vehicles and/or their reversing lights coming on. This would be particularly true if the driver was going against the usual flow of traffic.
I would expect that the insurance companies would agree that both drivers were partly to blame for the accident and pay out accordingly.
Chris