Can radiation therapy treat breast cancer that has come back in the lung?

Usually not. Breast cancer that has come back in the lung usually requires chemotherapy. It depends on the type of breast cancer, how quickly it is growing, and whether there are multiple spots or just one spot. Breast cancer in the lung means microscopic cancer has spread through the body, so a whole-body treatment like chemo (or hormone-blocking therapy) is necessary for best results.
Yes. If it comes back as just one lesion then radiosurgery is indicated. If it comes back more extensively then chemotherapy and or hormonal therapy is needed. In this situation radiation can be used to help any symptoms from the cancer that came back.
If it causes a. Symptom, it can relieve it, but it will not stop the appearance of oter cell clusters at remote locations, and should not be considered a "cureative" treatment, but moe a symptom relieving treatment, palliation.