My father is 80 years old and one of his carotid arteries is 90% blocked - is he a good candidate for surgery or not?

Possibly. It depends on his general health especially the condition of his heart and lungs. The surgeon must report no more than a 3 - 5% complication rate. He may be a candidate for a stenting procedure if he has heart problems or multiple strokes.
Will need to. Be evaluated to see if intervention is safe. I perform carotid artery stenting with distal protection on patients this age frequently. See stroke and carotid artery disease at www.Sirweb.Org for good info.