"...her car" está certo. "... Hers car" não.
Poderíamos empregar "hers" assim:
"She helped John clean his car and John helped her clean hers."
"Alice and John each own a car. His car broke down on the hill yesterday. So did hers but in the driveway a week later."
"The repair shop mechanics checked Alice's car for damage and John's for loose nuts and bolts. They didn't check hers thoroughly though."
"The repair shop mechanics told her her car needed a clutch job."