If odds really flipped by technical glitch, normally books, void the bet before start of the game or mail the user to give option of accepting new odds or void the bet. But here is situation is really different because of 50% cashout.

@FortuneJack was odds really wrong or it reduced continuously because of betting volume? If it was really wrong than you can void the bet technically even after user cashout the bet and revert the excess fund. But if  it was reduced so much because of betting volume than, EpicChamp deserve the that half bet stake back. Normally books offer goodwill gesture bonus in % of bet that was disputed, I saw you offered him 25% bonus, I think it's good if odds was really wrong. But if odds reduces because of betting volume than EpicChamp is right that his half bet must be refund.

Just 2 cent from my side.