Yes that can definitely be the case - that's what happened with me when I was diagnosed with breast cancer 13 years ago. I was found to have two small tumours in one breast which were quite far apart, and the surgeon told me it would be really difficult to maintain a reasonable breast shape if he did two lumpectomies and very strongly advised thst I had a mastectomy and reconstruction. I had a sentinel node biopsy before the main surgery, which established that the cancer hadnt spread to my lymph nodes. It turned out to be a really good decision to have the mastectomy as a third tumour was found that hadn't shown up on any of the imaging when they examined the breast tissue that had been removed.
I had expected that I would need to have chemotherapy and/or radiotherapy, but when I saw the oncologist, she explained that radiotherapy wasn't necessary because this would usually focus on the area around the tumours, but they had been removed altogether, and because my lymph nodes hadn't been affected, there was no need for chemo. They take so many things into account when deciding on whether or not you need chemo or radiotherapy, and although it's quite unusual not to need either, I'm proof that is does happen and thst there are really instances when it's not necessary.. I was treated at a leading London teaching hospital and was lucky enough to have had some truly outstanding senior surgeons and oncologists. Because my cancer was hormone positive, I took hormone therapy for five years, which I did find quite gruelling, and the recovery from the ten hour reconstruction also took quite a while.
My mum was diagnosed with breast cancer six months after me, when she was 85. She also didn't need chemo or radiotherapy - she had a mastectomy and was offered reconstruction, but didn't want to put herself through the ordeal of such major surgery. Because her cancer was hormone negative, she didn't even have to have hormone therapy.
I was a one to one support volunteer for Breast cancer Care after my treatment and I came across quite a few women in the same position who had had the same treatment as me over the years I worked for them. Oncologists now use more sophisticated testing to determine when chemo is needed, so yes, if your friend has been told that she doesn't need either chemotherapy or radiotherapy, that is entirely possible and she is one of the lucky ones like me. Don't worry, oncologists know what they're doing!