There is a major genetic component to fatty liver disease. In genomics, (genetics + how the genes are expressed in life) there are multiple genes that can code for an increased risk of fatty liver disease, especially in Quaker parrots. However, that doesn't mean that external factors (i.e. personal choices) cannot play an equally important role in developing fatty liver disease. Preventative health measures are the most important thing to remember. Eating healthy (low-fat diet), keeping well rested, and exercise and mental stimulation can all do wonders in keeping your parrot from developing fatty liver condition or other kinds of genetic diseases. Millet is for special treats and the occasional snack! Sometimes, bruising on the beak can be an indicator of fatty liver disease, but parrots are masters at hiding illness until it is often too late (they would be preyed upon in the wild if they show any signs of weakness or illness).
I'm very sorry about your loss, though. May Gus rest in peace, my thoughts and prayers are with you both