If she was in the store using the cafe etc find out the store managers name and ask them to cancel the ticket. You may well have to submit a copy of the till receipt. I manage a team who often have this happen, and in 99% of the times it happens we get the ticket cancelled.
It shouldn’t matter who manages it (unless it’s a town centre shared one) the Sainsbury’s Store manager can still send off a message and get it cancelled. Whether they choose do do so ,or say that they can’t, is another matter. Play the ‘I shop here, use the cafe etc regularly’ card, loyal customer and all that. If you park and then mooch off into the town centre you’re on a sticky wicket, but if you did stay on their premises spending money then you should find them more sympathetic. Speak to them sooner rather than later though.Where Sainsbury's own the car park there shouldn't be an issue - where they don't though the car park "could" be managed on their behalf.
In which case good luck. A number of M&S stores have managed car parks with particularly over zealous storm troopers on patrol.
Its managed by 'horizon' parking - seems like an independent company!
Do you still think its worth trying?
I hope she has the receipt... she normally keeps a pile of them..