The Idea Factory - Prison Heart Break
The main character has been in prison for a few years, but she is getting out in a couple of months and just wants to keep her head down and ride it out. At least, that was until she got a new cellmate. She galls for the girl hard, though the cellmate has ten years inside ahead of her. Only, the new arrival has no intention of staying that long. She has plans in place to escape within the week, going on the run and never coming back to the area. Without being able to reach out to her later, the only way that they can be together is if they both escape together.
In the end, though, part of the escape plan was to have an accomplice, who was never meant to survive the escape.