Alas the most likely reason for this would be that the user somehow returned to the front page and failed to notice that he had to reenter the discount code so that in the end he bought the ticket without the discount. Nearly happened to me on a few occasions. So yes, the proper resolution would be to cancel and then rebook. That the price went up in the meantime is just unfortunate. OTOH if the trip is not very soon, prices typically fluctuate up and down. So, tomorrow or the day after, might be down again.