Mother had tubular-lobular breast cancer and they also found a small area of dcis during mastectomy pathological analysis. is this common?

Yes: Complete pathology review following mastectomy surgery can often identify other small microscopic lesions. Dcis is a non-invasive early brsst cancer & would not affect the prognosis nor likely change her treatment plans. This is also an example of why additional treatment with hormone & radiation therapy is given if a person had only had a lumpectomy surgery.