I worked onc for several years. As to why your friend would think that renal patients were sicker and stay sicker, I really don't know. That assumption blows my mind, quite honestly.
We have patients who literally don't have a white cell to their name. Just because they're onc patients doesn't mean they're all compliant (not by a long shot!) or that they'll fight to stay alive (some people have been sick for so long, your job is simply to help them die in some semblence of peace). Not all of them are newly diagnosed, either, though just because someone is newly diagnosed doesn't mean they aren't symptomatic and very sick with cancer.
Oncology was intense. I often had patients who were a sneeze away from the ICU. It doesn't get much more acute than inpatient onc.
I can't speak for renal as I have never worked as a renal nurse, but I can't imagine anyone sicker than I've seen some oncology patients--who else do you know who is trying to fight off bacterial infections, viruses and cancer without an immune system in sight?