There is no specific formula for determining intangible damages suffered by an injury victim. The effects on a person's life are unique to each person.
One way that I like to help to break down my clients' intangible damages claims into smaller, more easy to grasp pieces is to discuss my clients' pain, suffering, etc. on a daily or hourly basis. If your father is in constant pain, then have him describe what it's like for one waking hour with pain. Use specific examples. Ask him to describe his first hour when he got up today. Ask him to describe the last hour before he finally got to sleep last night. Then, you ask your audience (adjuster, mediator, or jury) to assign a dollar figure for fair compensation for that hour of pain. Suggest a range to them, if you have one in mind. Is it worth $5.00, $10, $100? You can't put the jury in the Plaintiff's shoes; but you can ask what would an otherwise healthy person charge as a rate of compensation to take a job that entailed being in this kind of pain for every hour of his/her waking life?
Now ask your audience to do the math. Use a life expectancy table to get a number of years to start with. Break that down into days, then hours of pain. Multiply that hourly rate times the total number of hours in the life expectancy. They really add up. Now, be sure to remind your audience that the life expectancy is only an average. Half of the people your dad's age will outlive that life expectancy number of years. If your father was otherwise healthy and is economically fit so that he does not live in unsanitary conditions, then he may very well be in the upper half of the average; so those addtional years should also be taken into account.
As I am licensed in Florida and Vermont, I cannot offer you specific legal advice as to North Carolina state law. However, generally, the situation you described leads me to believe that your parents are not represented by legal counsel. That is a mistake in a circumstance like this. I would respectfully suggest that you encourage your parents to immediately contact a personal injury attorney in their area. Generally consultations are free and representation is based on a contingency fee basis, so there is no need to pay the attorney for his time as you go through a case claim. In the interim, your parents should not give any statement to the adverse insurance carrier nor grant them access to your parent's medical records. Remember, the adverse insurance carrier is not your friend nor your good neighbor. All insurance company goals are to pay nothing or as little as possible on any claim.
The best way to approach an insurance company for a potential settlement is through competent personal injury counsel who has prepared the presentation of all of the liability and damage issues, based upon his or her years of experience. There are many elements to a potential damages claim, including your mother's own derivative claim based upon your dad's injury. You may wish to review some of the Legal Guides which I have published on Avvo.com which deal with damages issues and others which you are facing.
I am licensed in North Carolina and I agree that this is something that your parents should talk to an attorney about. First, pain and suffering is difficult to quantify and therefore harder to predict what a jury might do. In my experience, the value of the pain and suffering will depend on the county filed (Wake Co. juries will give substantially less than Robeson Co. juries for the same case), the strength of the liability case, the strength or weakness of the cause of the injuries (caused by the accident or pre-existing), the type of injuries and medical bills (soft tissue injury with diagnostic treatments will cause a lower pain and suffering award than other types of injuries and bills for actual treatment or surgery). The settlement offer will also depend on the insurance company. Some companies like to pay early to save attorney's fees and some like to lowball people and gamble that overall they will save money. Don't believe the old "rule of thumb" that pain and suffering should equal 3x the medical bills.
Second, you are describing not only pain and suffering, but permanent disability. That is a different damage category altogether and depends on his activities before and after the accident as well as the above variables.
Third, auto owner's coverage in NC has a minimum limit of $30,000 per person, so if his damages exceed that, he may have to make a claim with his own underinsured policy. That is a pretty complicated statute and I would again recommend an attorney.
Feel free to call or email me if you have any additional questions.