Denies compensation or responsibility

My gran purchased a Defy DFF438 F790 Side-By-Side Fridge Metallic Fridge in November 2020. In February this year, she noticed rust behind the fridge. I informed the supplier, who contacted Defy, who then sent out technicians on three different occasions. They first claimed that the rust was either caused by damp on the wall behind the fridge or a burst geyser causing damp. However, there is no damp on the wall and the geyser is situated outside the unit. They are now claiming that as the back with the rust cannot be replaced (it is foamed in when the fridge is made), it is irreparable. They have stated that while they could not determine what could have caused the rust at the time of inspection, they cant rule out the possibility of water spillage on the unit. This is highly unlikely as my gran is 77 years old with a hunched back, and reaching the top of her fridge to leave water that might have spilled is improbable. Defy is not offering any compensation and is denying any responsibility, despite the fridge still being under warranty.