How would you manage a patient with early-stage invasive ductal carcinoma with associated low-grade DCIS who was found to have ADH at the tumor margin on post-op pathology?  

Presume it is focal and not diffuse involvement by AFH, I would get pre RT mammogram and if no residual calcification or abnormality, would proceed with RT.

Since the pathology is based on an excised specimen (lumpectomy), I would ignore the ADH. It is a benign process and of no relevance to your management of the patient - assuming all appropriate clinical/imaging standards have been respected, i.e. all concerning Ca++ removed.