I broke my Tib and Fib back in 1994 when I was 44. Just before I was due to go into Theatre to have it pinned, the surgeon took a close look at my x rays and decided not to pin it, but to let it heal naturally in plaster.
I was in a hip to toe plaster for 6 weeks, then an above the knee plaster for 4 weeks and finally a below the knee to toe and "shoe" for the rest of the time. In total I was in plaster from June until November.
I had physiotherapy after that and had to do exercises every day. My ankle was fine by Xmas but a lot of hard work was needed to get it back to normal.
I think I was lucky not to have pins and plates but with physio and exercises, your Mum should have full use of her ankle.
I wish her well.