Guilty Wives by James Patterson

Guilty Wives - Abbie Elliot and her three best friends are in Monte Carlo on a girls only trip.  While there they meet up with some men who have invited them back to a boat.  In the morning they are arrested for murdering the French President.  Knowing they are innocent they refuse to confess to the crime and after months of being in jail they are convicted. Abbie is incarcerated in one of the worst prisons in the system and is severely abused.  Even while there, warden continues to get her to confess, having her tortured when she does not confess.  Eventually, she understands who framed them and gaining help from an unexpected source, she escapes.