My father is 80 years old. He has a carotid artery that a screening has shown to be 90% blocked. Should he consider surgery?

Benefits vs Risks. Generally any blockage greater than 70% or those with symptoms of carotid artery disease should be surgically repaired. However given your father's age any pre-existing medical problems should be taken into account prior to surgical intervention. A vascular surgeon and your father's primary physician can collaboratively decide if the benefits of surgery outweigh the risks.
Depends on health. If he is healthy and active, then this may prevent him from losing that lifestyle due to stroke. If he has multiple chronic diseases and poor functional status, then risk may outweigh benefit. Before surgery he would need thorough eval. Vascular surgeon and anesthesiologist can advise.
Carotid Artery Block. You should obtain at least a couple of opinions related to this great question. A blockage alone is not necessarily a reason for surgery. Several major issues to ponder: what other morbidities does your father have (would this or any surgery be safe?), has he had any symptoms related to the blockage (strokes, tias?), and what is the expertise (the complication rates) of the surgeon(s) in your area.
Talk to doctor. If good function may need stent or surgery. Do medical treatment as long as possible.