The current thinking in breast cancer treatment is to be as minimally invasive as is deemed safe so where once a mastectomy would have been the only option, the surgeon is likely to go for lumpectomy with sentinel node biopsy if it looks to be containable, then reassess the situation should things look different under surgery or following the pathology report. I'm guessing that this is what is happening here but ask your Mum's surgeon to be sure. It might be best to remind people that your Mum is acting on the best current medical advice and that much has changed about bc treatment in recent years.