Stratum Benefits wont pay for a valid and authorised critical surgery

I am shocked at Stratum Benefits. My 78 year old father suffers from a condition called Dupuytren's contracture, which has resulted in his finger being permanently bent back towards his palm. He endured it for years but it was getting worse and worse and he could no longer function - he was unable to hold a s**** or fork or pen or glass. It was affecting his ability to eat and work (one finger on each hand are both bent right down). So he finally had no choice but to go for surgery. Fortunately, by Gods grace, the surgery was successful. Medical Aid paid but the gap cover company, Stratum Benefits refuses to pay. They say this procedure was cosmetic surgery. How can it be cosmetic surgery when my Dad was unable to write or hold cutlery? Did Stratum expect my Dad to give up his job, and become an invalid dependent on others to do every day tasks? They took his monthly premium for years, but now refuse to pay on a flimsy and medically incorrect reason.