No, it is completely inappropriate. In a family setting, especially in a confined space like a car, any behavior that is of a sexual nature is unacceptable. There are social and moral norms that should be followed, and teens should be aware of proper behavior around family members.
Sharing a story like that is completely unacceptable. A teen girl's sexual exploration is a very personal matter. It should be respected as private. Public discussion of such things can make the girl feel embarrassed, ashamed, and can also have a negative impact on her mental health.
Tell the person sharing the story that it is inappropriate and disrespectful. Then, try to protect the privacy of the teen girl in any way possible. For example, if it's in a group setting, change the topic to something more positive and respectful.
No. Such a topic involves inappropriate and private behavior that is not suitable for public discussion, especially in the context of a classroom which should be a place for learning and appropriate behavior.
There could be a story where a teen girl pays for minor repairs on her old, second - hand car. She learned about car maintenance from her dad and decided to use her part - time job money to fix a small dent or a broken side - mirror herself.
These stories are not that common in the sense that most teens don't go around sharing such personal experiences. However, in today's more open - minded society, there may be some who are more comfortable talking about sexual exploration in general. But still, when it comes to a teen girl's first time masturbating, it remains a very private and individual experience for most.
Gratitude. Maybe the mom has sacrificed a lot for the teen. She could have worked multiple jobs to provide for the family. So the teen, realizing all her efforts, decides to buy her a car as a thank - you gift.
She could try to gather evidence against him. If it's a fantasy world, there might be some magical or technological way to record his blackmail attempts. Then she can use this evidence to expose him and get help.