If the person wants to cheat, let them do it in single player, but once we get MP, then it's something we should get in a tizzy about. As long as they do it against the AI, there's really no harm or foul. The AI needs an overhaul anyway. But cheating against another human is just wrong.
As for modding, the benefits of my own mod are Global. 30 turn Golden Age, unit stats raised, HP modded(2,4,6,8 respectively). So mine doesn't benefit any one player, it's global, so I don't consider it cheating.
If a person dishes out the $50 for the game, they should be able to do whatever they want to it