My friend says there was an emperor of the United States. What's the story with that?
That would be Norton I, self-styled Emperor of the United States and sometime Protector of Mexico.
Joshua Norton (c. 1819–1880) was a businessman in San Francisco who made a fortune playing the real-estate market... and then lost it all on rice in 1854. In 1858, he went bankrupt.
In 1859, he declared himself emperor, sending the proclamation to the San Francisco Bulletin.
The question of whether he really was an emperor is largely philosophical. It is true, on the one hand, that he had no authority for declaring himself emperor, and that his various decrees to Congress and the army were completely ignored. So it would seem pretty clear he was not, in fact, emperor of the United States.
On the other hand, it's also true that the city fell in love with its eccentric "emperor" and largely indulged him. He spent his days walking the streets of San Francisco in a military uniform inspecting the running of the town, arbitrating disputes, and attending lectures and plays. He was given free meals at restaurants, which put up plaques declaring that they existed by appointment to the emperor. He and his two dogs had seats permanently reserved for them at theatres. He printed his own money, which was actually accepted by local businesses.
In 1867, a new police officer arrested Norton to be treated for a mental disorder. There was a public outcry, and he was released the next day with an apology from the police chief. Thereafter, police officers saluted him when he passed on the street. The census of 1870 listed him with "emperor" as his occupation. And when he died, virtually penniless, in 1880, between 10,000 and 30,000 people attended his funeral.
Perhaps, then, it's reasonable to say that while he may not have been emperor of the United States, he was the emperor of San Francisco.