Refund/Exchange denied and treated badly infont of costumers

My sister bought Simba chips and BP garage in Evaton north Pick and Pay express. When she got to the car the 125g packet was not even above half. When she returned to the cashier to show them and ask for exchange or refund, was told that it's not their problem and she chose that packet so she must live by her choice. She was so humiliated and treated badly.