Situation: A friend (John) traveled to Mexico with his family for vacation. John recently renewed his passport, but when checking in for the flight inadvertently used his old passport. Upon landing in Mexico he was denied entry for having an invalid passport and had to use ticket of his original return flight to go back home. After retrieving his valid passport he had to buy a new ticket to go back to Mexico and continue his vacation.

Question: Does United bear some of the responsibility for his return to Mexico for allowing him to check in and board with an invalid passport (effectively negating his original ticket)?

John is content with this being water under the bridge, but I was curious who is legally on the hook for any out of pocket costs when someone doesn't have a passport or travels on one that is invalid.