From the pics at link, it appears that Abe’s son may have been at his side. If so, the son witnessed the assassination of his father. Hope I’m wrong, but scroll down and look at pic in link.
The shooting captured on video
https://twitter.com/maike809588/status/1545255792902905856?t=4yUlIDwuMspg7KOChf936w&s=19
Robert Todd Lincoln was not at the theater when his father was shot, but was at his bedside when he died. He was serving as Garfield's Secretary of War and was present when Garfield was assassinated in 1881 and was nearby at an exposition when McKinley was shot in 1901.