-Radiotherapy is localized treatment, chemotherapy is systemic treatment. Radiotherapy is generally not indicated after a mastectomy because there's really nothing left to irradiate. With a lumpectomy
(which is what your mother was originally scheduled to receive) irradiating the surgical area to kill any remaining cancer cells is standard procedure.

I'm just guessing here, but if the surgeon didn't get clear margins when removing the tumors (a clear margin is an area of cancer-free breast tissue surrounding the tumor and a good indicator that all localized tumor tissue has been removed) then the next logical step would be a mastectomy; and since in most cases there's really nothing to irradiate after a mastectomy it's generally not offered as an option.

If your mother's oncologist is recommending chemotherapy it's because one or more of the characteristics of your mother's disease indicate chemotherapy - that could be tumor size, tumor type, evidence of local or distance metastasis, axillary or other lymph node involvement or something else.

-RT won't be needed although no clear margins were achieved?! Normally this is an indication to re-operation but after mastectomy its usually up to radiotherapy to clean up the mess.. The lymph nodes are by definition not involved in in-situ cancer, so I see no reason for prescribing chemo. Other indicators for chemo (with invasive ductal ca) are premenopausal age and lack of hormone receptors in the tumor. Perhaps the onc does not place much faith in that pathologists report? Or he knows about mets that he has not mentioned. See what another onc says, and if he comes to the same conclusion ask him to explain the reasons for chemo more clearly..