Barebone was a descendant of Scourers who believed that all witches and wizards should be killed. He befriends a gullible young witch named Dorcus Twelvetrees, plies her with seemingly innocent questions, then steals her wand and publishes the locations of both Ilvermorny and MACUSA headquarters. Dorcus serves a year in prison for her crime and MACUSA is given a severe and embarrassing reprimand by the International Confederation of Wizards (Pm).
The affair between Dorcus and Bartholomew results in a great uproar in the wizarding community, after which Dorcus serves a year in prison. During that year, Rappaport's Law is passed, which we know happened in 1790. Working backwards, assuming the speed with which news and information would travel in the late 1700s, we estimate that the relationship developed approximately two years previous in 1788, and that the betrayal by Bartholomew would have happened during 1779.