Buying a seat on an airplane is not like buying a loaf of bread. It is accepting an offer to transport your according to the terms of the Contract of Carriage:
People can make up all the crap they wish to, but the terms are spelled out, and as I stated before this is more about PR and damage to the UA brand than actual damages. He will have to sue and show how this situation requires a remedy that goes behond the terms he agreed to:
https://www.united.com/web/en-US/content/contract.aspx
He was seated on the plane. United had an obligation to take him. They can’t pull the overbooking stuff at that point.