I think the easiest way is to give up smoking. Then the yellow will not increase in your father's eyes. But I still think that you can take your father to have an eye exam in the hospital, because many diseases will cause yellow eyes. If there are some infections in the liver of a person, he or she may have yellow eyes too.
Dear friend, if your father's yellow eyes are really caused by smoking. You can ask him to reduce the amount of cigarettes and then try to give up smoking. Then the symptom will go away on its own. Remember to get enough rest and keep a balanced diet and exercise more. But I also heard that there are some very effective drugs for yellow eyes. You can ask your doctors to get more information about this.