Mary was a leaker

[Susan Higginbotham]

The story of Lincoln's intervention dates back at least to 1886, though, when Ben Perley Poore recounted it in his memoirs.

"The House accordingly directed the Sergeant-at-Arms to hold Wikoff in close custody, and he
was locked up in a room hastily furnished for his accommodation. It was generally believed that Mrs,
Lincoln had permitted Wikoff to copy those portions of the message that he had published, and this opinion was confirmed when General Sickles appeared as his counsel. The General vibrated between Wikoff 's place of imprisonment, the White House, and the residence of Mrs. Lincoln's gardener, named Watt. The Committee finally summoned the General before them, and put some home questions to him. He replied sharply, and for a few minutes a war of words raged. He narrowly escaped Wikoff's fate, but finally, after consulting numerous books of evidence, the Committee concluded not to go to extremities. While the examination was pending, the Sergeant-at-Arms appeared with Watt. He testified that he saw the message in the library, and, being of a literary turn of mind, perused it ; that, however, he did not make a copy, but, having a tenacious memory, carried portions of it in his mind, and the next day repeated them word for word to Wikoff. Meanwhile, Mr. Lincoln had visited the Capitol and urged the Republicans on the Committee to spare him disgrace, so Watt's improbable story was received and Wikoff was liberated."

See vol. 2, pp. 142-43.

https://archive.org/stream/perleysremini...2/mode/2up

I do think it's possible that Lincoln privately spoke with the Republicans on the committee, as this account suggests.