If he honestly believes that he did everything right, and he prepared as well as he could and his body was right before the fight, then he only has 2 options:
(1) Switch camps. If you did everything right, then your camp (environment) is failing. This is the Universe telling you that your current trainers have done all they can do for you. Now, it is time to adapt and survive with a different camp and environment. It is time to be an uncomfortable hungry fighter again. To quote Jordan Peterson, it is time for the lobster to shed its current shell, so it can grow again.
Leave Las Vegas and move to AKA. Why AKA? It is a team of fighters who can actually challenge and help him develop his "TD, back take and RNC" approach to fighting. Kevin Lee could win a lot of MMA fights just pinning opponents to the fence, and getting into dominant position.
Kevin Lee can beat the majority of LWs and WWs the same way Alxander Volkanovski just beat Jose Aldo. Stay calm. Avoid the KO shot. Pin opponent against the fence , because you are scoring points with the judges while not even using your gas tank.
Just take dominant position, and pretend to be active until the referee says something. If the opponent gives you an easy sub, then take it. He hasn't had any problems doing this against most of his opponents. AKA can really help him develop this skill.
(2) Retire and save his brain. He has already accomplished (in his profession) more than the average American will ever accomplish in their professions. If he has children, and tells them that he was a UFC MMA fighter who fought for a belt. If he treats his children right, then they will always believe that they have the greatest and most bad-ass dad in the world.
If you know Kevin Lee, then pass this along to him.