According to divorce suit by legal husband Henry Cobb, Augusta Cobb (plural wife of Brigham Young) committed adultery with Brigham Young the first time on August 10, 1844 in Boston, and a 2nd time in Nauvoo on December 1, 1845.
Court testimony provided by Elder George J. Adams states she then told Henry she loved Brigham Young better than she did Mr. Cobb, and, live or die, she was going to live with him at all hazards. This was in the course of a conversation in which she used extravagant language in favor of Mr. Young, and against Mr. Cobb.
"To the Honorable the Justices of the Supreme Judicial Court now holden at Boston within and for the County of Suffolk."
"Henry Cobb husband of Augusta Cobb of said Boston, libels and gives this Honorable Court to be informed, that on the twenty fifth day of December in the year Eighteen hundred and twenty two, at Charles town in the County of Middlesex, he was lawfully married to the said Augusta Cobb, and hath always behaved towards her as a chaste and faithful husband; yet the said Augusta neglecting her marriage vows & duty, since the said marriage, on the tenth day of August in the year Eighteen hundred & forty four, at said Boston, and on the first day of xxxxx December in the year Eighteen hundred and forty five, at Nauvou in the State of Illinois, committed the crime of adultery with one Brigham Young. Wherefore your libellant prays that the bonds of matrimony may be dissolved between himself and the said Augusta Cobb"
[Marquardt Papers, Suffolkss Supreme Judicial Court]