You are asking the right questions. It's not clear how this will play out but the passenger in this case has sued United asserting a variety of theories - negligence, emotional distress etc. There will be an argument that an airline owes a duty to its invitee. There will also be a question as to whether it is negligence to allow an intoxicated person to board. On the other hand the intentional actions of another (assuming they are found to be intentional) may serve to insulate UA. It promises to be an interesting case. And I think that UA refused to identify the other passenger prior to suit being filled - but clearly they know who he was and will be forced to identify him now that suit has been filed.