Talking about masturbation in general can be a very private matter. In the context of so - called 'friend masturbation stories', if it's shared inappropriately, it can make others uncomfortable and violate privacy norms. However, if it's in a scientific or educational discussion about healthy sexual development among friends in an appropriate setting, it could potentially lead to better understanding of sexual health.
One should firmly but politely let the friend know that this is an inappropriate topic. For example, you can say 'I don't think this is something we should be talking about. It's very private.'
No, not in general. As long as they are private and not interfering with one's daily life or relationships, they are a normal part of human sexual expression.
No. Teaching masturbation stories is completely inappropriate and violates moral and ethical norms. It involves private and sexual topics that are not suitable for public discussion or teaching.
It depends on the context. In a private, consensual conversation among adult friends who are comfortable with the topic, it can be appropriate as it's a form of sharing and communication. But in a more public or professional setting, it's usually not appropriate.