The “rubber box” or did Laura Keene indeed held Lincoln’s head?

[RJNorton]

Actor E.A. Emerson, who played Lord Dundreary in Our American Cousin, recalled that she (Keene) "was one of the first to reach the box, and when I saw her she was holding the president's head in her lap and the handsome yellow satin dress she wore in her part was stained all down the front with his blood."

Is there a claim that Emerson was also in the (ever-expanding) box? If not, where (if anywhere) could he be standing to be an eyewitness to this?