I am not a lawyer, but I understand what can happen in consumer abuse cases because my father was in the middle of one that is somewhat analogous to this one. This was in the days prior to cell phones and social media, so the problem for United is much larger now.
My father would have played the role of poor Dr. Dao. His situation was as follows. He checked into a Hotel/Casino in Nevada for the night and was given a discount coupon for one of the restaurants. My father ate dinner and attempted to pay using the coupon which was NOT honored by the restaurant staff. The staff called security who took my father into a back room and roughed him up (injuries were no where near as severe as poor Dr. Dao's). They made him cough up the money then made him sign a statement saying they had done nothing wrong. He gladly signed it knowing it would never hold up in court because the signature was obtained under duress.
The next morning he went to an attorney who did a little discovery and found that the practice of denied vouchers was widespread. The subsequent class action lawsuit put the Casino out of business.