My mother was a victim of a fraud and Barclays UK refuses to return the money

Hello guys, i am really in need of help. My mother is working in the UK, she has basic knowledge of the language , and recently someone stole from her account 4000 pounds. The transaction was made from her internet banking system. Before this, she received an sms from the bank stating : " thank you for changing your phone number" although she did not request that. At the time she was in medical leave at home, as she broke her leg. After going to the bank they refused to give her a statement with the transaction/s. Now , after the bank investigation they said that she was the one that made the transaction and no refund is necessary. What is to do in this case? The bank did not inform her on nothing about the investigation , only that the transaction was made by her which is not true. She did not request to change her phone number or nothing. thank you in advance! The bank is Barclay's UK (London branch)