If a duel was accepted and then forfeited by one participant not turning up for the fight, I'd say it should be considered as won by the duelist who did turn up to the fight on grounds of cowardice by the other noble. Assuming there wasn't some obvious reason why the forfeiting noble didn't take part, e.g. if they were taken out by an assassination before the duel could be fought. If that was the case, the duel clearly couldn't have happened.