Join Date: Nov 2013
Location: NYC / TYO / RSW Up in the Air avoiding the Snowflakes
Programs: UA GS 1.8MM, AA 2.1MM, EK, BA, SQ, CX, Marriott LT, Accor P
Posts: 8,039
IIRC when you get a new passport now the old one is invalidated in the US system -- but the expiration date printed in the passport could easily still be valid and would pass any physical review...
Regardless -- "John" is responsible for having valid travel documents -- and Mexico only requires validity for the duration of stay -- not an extra 6 months like many countries...
I personally experienced this blunder years ago on a trip from Japan to Singapore -- but UA caught it at check in from Japan to Singapore.