My great-grandfather was a Confederate cavalry soldier fighting under Johnson's remaining forces after Lee's surrender. In his diary, he writes on April 18, 1865, "Good news today. Lincoln has been killed while in his box at theater. Washington and Bill Seaward stabbed in several places." I'm not sure who this "Washington" person is, but I know that the assassination of Lincoln and the attempted assassination of William Seward occurred on April 14, 1865 -- just four days before the diary entry. I assume that there was no telegraph service between the north and south during the war. How else did news from the northern states, such as Lincoln's assassination, make its way to the people of the Confederate states and its soldiers in particular?
As soon as the death was published in northern newspapers it would have become available to the south. For an important event, like Lincoln's assassination, a man would have used a horse and carried a newspaper right to Richmond, which is about 100 miles away from Washington DC, where the assassination occurred. Since the first reports were published on the 16th of April, they would have been printed in Richmond on the 17th of April and the next day, the 18th of April all of the newspapers in the south would have published the news, having received it by telegraph or courier.