he has every right to disobey the order he was given. he paid for his flight, UA overbooked, that's their problem, not his. UA have no right to tell any passenger to disembark when the passenger has done nothing wrong. and he did nothing wrong.
he initially agreed to help them out by taking the offer of $800 and a later flight, but when he went to disembark he was told the next available flight would be the next day, so he changed his mind due to him being a doctor and having patients to see the next day, he has every right to change his mind if UA were not forthcoming with the exact details of their offer.
and employees are not "just doing their jobs" when they physically manhandle customers. If a customer was being physically threatening to staff or other customers, fair enough, but when a customer is complaining, there is no recourse for physical aggression on the part of the company.