You give the Cashier's Check to the Body Shop to fix your car.....Why would you "need the money in your account"?
Go through with the repair and learn to negotiate better. The check directly to the body shop is a common practice from insurance companies anyway. If you think she did it in bad faith, enlist the help of a lawyer.
Tell her to send another check. She didn't make it out to the right person.
She has to