Gems refusing to pay for retired member's cancer treatment!!

My mum retired in December 2019. She has been covered by Gems for many years and is a breast cancer / chemotherapy patient, since 2017/18. My mum, soon after retirement has been subjected to paying thousands of rands by Gems, who refuses to pay for her treatment and can't track down the excessive extra amounts that she has been paying in 2020. She has now missed two cancer treatments and can't continue her treatment because it keeps coming up that she owes money! What is going on here?!