West Whiteland police say 27-year-old Christina Borden threw the 16-year-old victim to the ground in late September.
She then is accused of repeatedly striking him with her knee, breaking his arm.
According to investigators, Solgie Barbar and Monique Scott witnessed the incident but didn't report it.
They are required to do so by law.
Borden failed to post her cash bail, so she is in prison.
The other two have been released on bail.
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