Assassination Trivia

[RJNorton]

One person who knew Booth noted the change of appearance in how JWB was dressed during the afternoon and when he entered the theater to shoot the president. The person noted that in the afternoon JWB "was dressed in the height of fashion, in the picturesque costume of the day, velvet collar and cuffs." But just before the assassination this same person observed that JWB "was wearing heavy riding boots, spurs, a blue flannel shirt, and an army slouch hat."

Who was this person?