I think if he ate a plant prior to accident that wouldn't have caused pneumonia. Once he was in a coma he could have inhaled dust containing plant material.
"Pneumonitis is a term that also refers to inflammation of the lungs. It may be due to infection, an allergic reaction (caused by inhalation of dust containing animal or plant material), exposure to radiation, accidental inhalation of vomit or other liquid (aspiration pneumonia), or as a rare side effect of certain drugs (acebutolol, azathioprine)."
http://www.nmihi.com/p/bacterial-pneumonia.htm
"Pneumonitis is a term that also refers to inflammation of the lungs. It may be due to infection, an allergic reaction (caused by inhalation of dust containing animal or plant material), exposure to radiation, accidental inhalation of vomit or other liquid (aspiration pneumonia), or as a rare side effect of certain drugs (acebutolol, azathioprine)."
http://www.nmihi.com/p/bacterial-pneumonia.htm