OK, she's probably being considered as a "legacy case" if she's been here for 5 years or more. She'll have been asked to provide evidence about how well she has integrated into Scottish society.
As well as asking you for a letter, she should give her lawyer school reports, photos of her and her daughter "taking part" in things, details of any voluntary work she has done etc, etc.
Your letter should probably include things like what a good neighbour she is, how her daughter is more Scottish than South African, that she is accepted in the community and is prepared to help out when needed etc. It's basically a personal letter of support, saying that she is so integrated that it would be inhumane to send her home, and that it would have a bad effect on the other people living there (especially the children).
Hope this helps.