The fact that he turned someone in and they "turned out" to be guilty makes it very likely that we just don't have all the information here. Certainly not enough to condemn an accuser that was right.

Are citizens required to follow rules of probable cause before reporting someone to the authorities?

You didn't read the original article. He claimed to be being picked up by his chauffeur. According to this guy anyway.

You are very mistaken about the legality here. Probable cause only matters to law enforcement agents. It has no bearing on private citizens.

The private citizens rights to seize property, turn it over to LEOs, report someone, conduct a search, do not change if they have probable cause to believe a crime has been committed. So, if I'm your boss and can legally read your emails, I can just turn them over to the law enforcement agencies no question asked – they're "my" emails after all.