Unhappy Loyal Customer

My niece bought clothes from Jet Menlyn park a week ago and one of the items was too small and getting an exchange seems impossible as the store manager isn't willing to assist or hear anything. My niece works in JHB and due to myself losing the receipt the manager refused to help with an exchange even though I showed him Proof from the bank that the item was bought in that store. How can such a reputable Store like Jet treat customer's like that. I would like to take these up with Head Office as the manager refuses to listen and assist me.