Originally Posted by RobOnLI

"John" could have boarded with one of those picture-taking boarding gates where they don't even bother asking for or looking at your passport. I had this several times on European flights ex-IAD. I could have landed in FRA/LHR/AMS/etc. without a valid passport and UA would have been fined a lot more heavily than to Mexico (my guess).

This is part of the reason why these picture-taking boarding gates are an awful idea unless they are used in conjunction with checking physical passports for validity.

To the OP: UA owes him nothing. UA will already be fined for transporting somebody clearly inadmissible to Mexico but it's the traveler's responsibility to make sure all documents are in order and valid.

-RM