Did US check to see if there were any company employees occupying a seat? If thats the case this man should have been given that seat. Second he should be at minimum refunded for his entire trip. As an industry employee of 30+ years I find these kind of corporate decisions ill advised and very damaging to what is a great industry.
Hire the right people, trained them to prevent these type of issues and if mistakes are made, which there will be, recover in an equitable and satisfying manner.