Okay .. JK, the purpose of radiotherapy in the adjuvant (curative) setting is invariably to prevent local recurrence. In the palliative setting it is about disease control.
There are different types of breast cancer, for which different management is indicated. If, for example, your friend had a tiny DCIS, it could be managed by WLE (wide local incision) only, and chemo / radio would unlikely be indicated.
If your friend had a triple negative cancer (oestrogen / progesterone receptor negative, HER2 negative disease) then all forms of targeted endocrine therapies would be redundant and chemotherapy / radiotherapy would be her only options at controlling disease or preventing its recurrence. Ergo, radiotherapy would be invaluable in such a case.
If I knew more, I could give you more specific, detailed information.
Do you know how advanced your friend's disease is? Has she been given a prognosis? Did she have surgery to remove the cancer? Has she had any further tests such as Oncotype DX?