Is the gushing of blood from the neck of the deceased, hours after his death, along with the observation of a light on his face when he was placed in the grave, considered among the signs of martyrdom, especially since he died "mabṭūn" (of an internal disease)?

May Allah have mercy on your friend. As for liver and kidney disease, they may fall under the category of an abdominal illness, the sufferer of which is considered a martyr. The one who dies from an abdominal disease is a martyr (al-Mabtoun), and this includes liver cirrhosis. As for the whiteness and luminosity of the face, it is a good indicator, among the signs of a good ending.