Contract null and void

My mom signed an Exclusive Sole Mandate on the 14th December 2020 for her flat to be sold. While I was on leave I searched the internet to see if I could find the advert for the flat - nothing, and no sign up outside the flat. Another buyer came forward on the 6th January 2021 with cash. I messaged the Property24 agent and told her. She told me that due to the contract, my cannot sell through anyone other than them. I asked her where she had advertised the flat and she said that the website was faulty and the sign had been ******. My IT specialist started browsing and found the flat advertised at about 11am on the 7th January 2021. This is a breach of contract on the agents side as she has done nothing to sell the flat even after being told it was urgent. They don't even deserve one star. They have handed the matter over to their attorney and mom received and email from them saying she is not allowed to sell the flat until the mandate ceases on the 30 April 2021.