In my experience, it can be good. Like when my friend and I shared a mutual friend who was a girl. We all had a common interest in music. We would go to concerts together. My friend and she got along great, and it made our group more fun. There was no jealousy or bad feelings because we were all just friends having a good time.
If you don't want to be so direct, you can try to change the topic whenever he starts. Maybe bring up a new movie or a book you've read recently. This way, you can steer the conversation away from his gf stories without hurting his feelings.
One possible reason for this going wrong is a lack of clear relationship boundaries. If the girlfriend, boyfriend, and best friend don't clearly define what is acceptable behavior, confusion can occur. For instance, if the best friend starts getting too involved in the couple's affairs. Another factor could be the best friend's own insecurities or loneliness, causing her to overstep. The key is for all parties to respect the main relationship between the girlfriend and the boyfriend.
There would be a lot of jealousy. Both the best friend and the person sharing the girlfriend might feel envious of each other's time with her. It could also lead to a breakdown of the friendship. If there are any feelings of inequality in the 'sharing' arrangement, it can create resentment.
In most cases, it's normal to share some stories now and then. But if it becomes the only thing he talks about, it might not be so normal. Maybe he doesn't have other things he feels comfortable sharing, or he's just really into his gf at the moment. However, it's important to communicate if it starts to bother you.
The most interesting part could be the origin story of their friendship. Maybe they met in a really unusual way, like they were both reaching for the same book in the library at the exact same time and from that moment on, they just clicked and started this amazing friendship journey.
I haven't personally heard of a positive 'sharing gf with best friend' story. In most cases, it would lead to a lot of drama. For instance, the two guys might start fighting over the girl's time and affection, and the girl would be stuck in the middle, feeling confused and pressured.
My friend and I used to share interests in girls. There was this one girl we met at a concert. We both thought she was really cool. So, we approached her together. We became a group of friends first. Eventually, my friend started dating her, but I was okay with it because we had a great time all hanging out together, and it was a really positive experience for all of us.
Sharing a wife in the context of stories is also inappropriate as it objectifies the woman. In modern society, we should uphold the equality and dignity of every individual in a relationship. A marriage is a sacred bond between two people, and such ideas are not in line with the values of a healthy relationship.