The prison guards in their zeal to recapture the escapee grabbed the first person they found in the vicinity of the accident. The only trouble with that action was that the replacement was a young boy.
He was unsuccessful in convincing anyone in the search party that he wasn't a girl and definitely not Rebecca Conyers. The searchers found him near Becky's discarded prison uniform and fetters and the conclusion at the time was that she was trying to use being a boy as a ploy to avoid incarceration.
No one bothered to check his story and he was carted off to the women's prison.