I'm not sure specifically as there could be many novels with that title or something similar. It might be about a great girlfriend's story, perhaps her adventures, relationships, or personal growth.
Definitely not. These types of stories go against basic moral and ethical principles. Friends' relationships should be respected, and creating or spreading such erotic stories about their girlfriends is a form of betrayal and inappropriate behavior.
One sign is secrecy. If she starts being very secretive about her phone or where she's been, like always taking her phone to the bathroom or not sharing her location. For example, in a story I heard, the cheating girlfriend would never let her partner see who she was texting.
One story I heard was that a guy noticed his girlfriend was always on her phone late at night. When he finally got a chance to look, he found flirty messages with another guy. She had been making plans to meet up with this other person behind his back.
One sign could be that she becomes overly secretive about her phone. For example, she always takes it with her everywhere, even to the bathroom, and gets very defensive if you ask to see it.
A bf and gf lived in a small town. The bf worked at a local store and the gf was a teacher. Every day, the bf would wait for the gf after school with a small flower. They would then go to their favorite diner and share a meal. On weekends, they would drive to the countryside and have picnics. Their love was simple yet very strong, based on these small daily acts of kindness and shared time.
One anime tomboy gf story could be about a girl named Miki. She has short hair and always wears boyish clothes. She falls in love with the main male character, Taro. Miki is really into sports, especially soccer. Taro at first doesn't notice her as more than a friend. But as they spend more time together in the soccer club, he starts to see her in a different light. Eventually, they start dating and Miki's tomboyish nature makes their relationship really fun and unique.
No, it is completely unethical. Being involved in any sexual relationship with a friend's girlfriend violates the basic principles of friendship and loyalty. It can cause great harm to your friend and lead to the breakdown of the friendship.