Abdominal Recti Post Nephrectomy?

This is not diastasis recti, but it is similar in that the muscle wall on the side that you had the nephrectomy on has probably been weakened from the surgery. This can be a true hernia or just a weakening of the muscle wall. It is possible to tighten it if it is loose or repair it if there is a hernia and this will likely improve your bulge. Of course, you would need to be seen in person to evaluate your abdominal wall to know for sure. 

I am sorry to hear about the problem you are experiencing after nephrectomy surgery. Your description is most consistent with either a true hernia (defect in the abdominal wall fascia) or laxity of the fascial tissues.    Either way, unfortunately, exercise is unlikely to improve the situation. Diastasis recti refers to spreading of the rectus muscles that commonly occurs during pregnancy and with weight gain.  Re approximation of the rectus muscles  is commonly performed during the tummy tuck procedure. You may wish to seek in-person consultation with well experienced board-certified plastic surgeons for accurate diagnosis and treatment recommendations. Best wishes.