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Was your brother already a transplant candidate at the time he booked the ticket? If so, did he consider booking a refundable ticket? Did he purchase the ticket with a credit card that offers trip-cancellation insurance? (Not sure whether such insurance would apply to pre-existing medical conditions.)

If the ticket was cancelled, didn't your brother receive a credit which can be applied to future travel? Or did he purchase a cheap, "Basic Economy" fare, which AA warns is non-refundable/non-changeable?