United Airlines had nothing to do with beating the poor doctor

[The Velveteen Ocelot]

82. Flights on United Express are listed as United flights.

When you go to United's web site to buy a ticket and request a flight from ORD to LEX, you'll see, in small print, "Operated by SkyWest as United Express" or "Operated by ExpressJet Airlines as United Express," or whichever other regional carrier operated the flight under contract with United. The listing includes the United logo.
https://www.united.com/ual/en/us/flight-search/book-a-flight/results/rev?f=ORD&t=LEX&d=2017-04-17&r=2017-04-19&sc=7,7&px=1&taxng=1&idx=1

When you get to the airport and go to your gate, the airplane you see will be painted with United colors. Since United is essentially representing the flight as being theirs, they will be responsible for it. I am assuming United will go to the regional airline and demand they retrain their staff; they could also terminate their contract with them, although that's doubtful. Of course some of the consequences will trickle down to the actual operator, but it's pretty hard to wiggle out of a situation that you have represented to the public as basically owning. The lawsuit will most likely name United, United Express d/b/a Republic, and the Chicago police and they'll have to sort out the liability among themselves.