Callie Brown, now 17, questioned how Karen could be so “cruel” to her own daughter.
She told a national newspaper: “I wanted to really punch Karen. How can your own mother do something so cruel to your own daughter?
“It’s not natural, it’s unreal, it’s disgusting really.
“We took Karen into our own home and took her in as one of us, and for her to do something like that and for the truth to come out.
“It was just horrible.”
Karen Matthews became notorious in 2008 after Shannon, then nine, disappeared from their home in Dewsbury, West Yorkshire, only to turn up 24 days later, hidden in her stepfather’s uncle’s home, less than a mile away.
Matthews and Michael Donovan were jailed after a court heard that they planned the disappearance in an attempt to claim a cash reward.
Callie said it had broken her heart to lose Shannon, who has since been moved to a new home and given a new identity.
She described her childhood friend as “kind and caring”, adding that she got on with everyone.
Callie appears as an extra in Moorside, a BBC drama - starring Sheridan Smith - based around the hunt for Shannon.