User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-1515612-20130221033240/@comment-3146930-20130225220406

Nico is allowed at Camp Jupiter because he told Reyna that he was an ambassador for Pluto, not Hades. They would have no reason not to believe him and the lars around Camp Jupiter wouldn't call him a Greek out of respect.