as she is not legally bound to pay them.
Technically that is true, it is a civil offence at the moment so there is no legal obligation BUT if the parking company take her to court she will then be legally obliged to pay.
Costs rapidly escalate if the case progresses further, you take your choice on the risk, a lot of cases are not taken further but one person ended up with a total of £24,500 in total CLICKY
after it went to court etc.
In reality if she is a regular shopper there and speaks to Tesco nicely they will probably get it cancelled.
I wouldn't volunteer information that she left the Tesco premises (went for coffee?) because in that case there is no 3 hour rule - she parked contrary to the contract.