Can a doctor refuse to treat a patient who takes cannabis?

A friend was recently diagnosed with multiple myeloma, a cancer of a type of white blood cell called plasma cells. There is often significant pain associated with this condition, and my friend has been treated with high doses of different opioid combinations, in addition to chemotherapy. My friend lives in Colorado, where he has easy access to medical marijuana, and he asked his oncologist if he could add this to his regimen in hopes of decreasing his dependence on opioids.