Oh, Sweetie, this is so tough for you. If your mom is not able to make a decision it will be up to you - and it all so much depends on other things. Where is the tumor, how large, etc. How is she physically otherwise? How frightened would she be afterward, if her breast were gone and she couldn't remember why?
As with all difficult decisions, get as educated as you can about the surgery itself and the implications for after-care. Write down pro and con lists. Pray alot. Then look at quality of life. It may be that they can just do a lumpectomy and follow-up with radiation. They can also do a test the day of surgery on the sentinel nodes and find out if it has already spread.
Whatever happens, dear Bridget, please know that you are on my prayer list.