I don’t know the details of this case but yellowing of the skin and mucous membranes, commonly called jaundice, is a symptom and not a specific disease in itself.
Normal bile pigment is called bilirubin and is formed as a result of the breakdown of haemoglobin present in red blood cells (RBCs).
If there is an increased rate of RBC breakdown, as can occur with a number of disorders, abnormally high levels of bilirubin will form.
These high levels of bilirubin cannot be excreted at a normal rate, and thus, accumulate in tissues.
Bilirubin levels may also increase in conditions where normal excretion of bilirubin is hampered due to some diseases in which bile cannot flow from the liver to the duodenum (first section of intestine) due to mechanical obstruction (such as an intestinal foreign body) or cancer.
Just to make things worse, bilirubin is toxic, further compounding the disease process.
Glad your friend’s dog recovered.