I had to do some digging but the article is a little fuzzy on a few details. The at-fault driver only had the bare minimum coverage and his insurance company only contributed $100k. The other victim, Duncan, also had uninsured-underinsured coverage and it was her policy that kicked in the additional $800k. Herrin also had at least one uninsured-underinsured policy which covered his son.
He collected $1.6 mil from his insurance company (perfectly fine since it was his and he paid for it) and a majority of the $100k from the at-fault guy's insurance (again, not a problem since he suffered the greatest loss). The problem was that he was also given a huge portion (over $677k of the $900k total pot) that included a huge amount the other victim's insurance policy.
It is my understanding that the whole purpose of uninsured-underinsured policies is to cover those who aren't insured. Since the at-fault driver's insurance didn't cover expenses for all the victims, it fell to each individual's insurance policies to make up the difference. It seems odd that he would be able to claim a portion of the other victim's policy in the first place.
Impossible to tell from the details provided, but I suspect the portion of the uninsured-underinsured he collected was from the driver's insurance policy. His son was a passenger in the vehicle that was struck, and the driver's insurance would cover the driver and passengers.