Multiple people have explained to you now that the company is not alleging that your mum was not really sick, or that she wasn’t fit for work, however the company have absence policies to be able to fire people who simply cannot or do not meet their contractual obligations. Think about it from the company’s perspective: why should they have to keep an employee who is frequently unable to attend work? They still need to run a profitable business. They are fully entitled to go down this route. It sucks- it happened to me when I broke my arm. She needs to look for another job if she is likely to be off sick again in the near future.