If you can think of two buddies in the restoration of the gospel that Joseph Smith did.
Oliver Cowdery is one of the names that usually comes up since Oliver Cowdery was present in some of the most important events in Church History.
According to the LDS church history records, he helped with the process of translation of the Book of Mormon.
He allegedly saw an Angel and saw the plates, besides that he was ordained to the priesthood by another Angel.
So why would such a key person in the restoration be excommunicated of the church?
I suppose that seeing an angel would be a good motivation to obey the prophet regardless of what he says and does.
Well, that was enough for Oliver Cowdery since he accused Joseph Smith of adultery
The Joseph Smith Papers Project says the following:
Church council excommunicated Oliver Cowdery, Far West, Caldwell County, MissouriOliver Cowdery Excommunicated
So it is a fact that he was excommunicated. The Encyclopedia of Mormonism confirms the excommunication and points out some of the reason why Oliver Cowdery was excommunicated:
The EOM says the following:
The Church court considered five charges against Cowdery: inactivity, accusing the Prophet of adultery, and three charges of beginning law practice and seeking to collect debts after the Kirtland bank failureCowdery, Oliver – Encyclopedia of Mormonism
Wikipedia provides additional details about who is the lady involved in the accusation made by Cowdery.
Wikipedia says the following:
In 1838, as Assistant President of the Church, Cowdery resigned and was excommunicated on charges of denying the faith. Cowdery claimed Joseph Smith had been engaging in a sexual relationship with Fanny Alger, a teenage servant in his homeOliver Cowdery – Wikipedia
In the Wikipedia page of Fanny Alger, we find more information about that problem:
In January 1838, some months after the Algers had left Kirtland, Oliver Cowdery—one of the Three Witnesses to the authenticity of the Book of Mormon—wrote to his brother concerning his indignation at Smith’s relationship with Alger.
Cowdery said he had discussed with Smith the “dirty, nasty, filthy affair of his and Fanny Alger’s … in which I strictly declared that I had never deserted from the truth in the matter, and as I supposed was admitted by himself.”
As Richard Bushman has noted, Smith “never denied a relationship with Alger, but insisted it was not adulterous. He wanted it on record that he had never confessed to such a sin.”
The best statement Smith could obtain from Cowdery was an affirmation that Smith had never acknowledged himself to have been guilty of adultery. “That,” wrote Bushman, “was all Joseph wanted: an admission that he had not termed the Alger affair adulterous.” In April 1838, Mormon leaders meeting as the Far West High Council excommunicated Cowdery, in part because he had “seemed to insinuate” that Smith was guilty of adultery.Fanny Alger – Wikipedia
If you want to read some of the charges presented against Oliver Cowdery, Wheat and Tares details these in a post called: