I was dating a UA FA who told me about two instances where she had to deal with a passenger dying mid-flight ... both times she and her colleagues were able to use an empty FC seat

the extreme case was the Braniff captain who suffered a fatal heart attack at the controls of their 747 about two hours out of HNL; his wife was the lead FA on that flight ... as I recall, the other FAs moved all the upper deck pax downstairs so she was able to be alone with him, and the flight continued to DFW