Medicine is a hard occupation. You always care about your patients but you have to keep your distance emotionally because death is inevitable. Especially in oncology. If it affecting her heavily she may want to consider another field in medicine with less death. How to comfort her, I don't know honestly. I've been in the Army and you repress much of what you treat, as well in trauma units. Since becoming disabled I have found blocking out all of that becomes more difficult and even PTSD surfacing.
In retrospect helping people is very fulfilling, but so is being a florist or a potter with much less death involved.