If she does it as a form of joking, try to find other ways for her to express her humor. Maybe suggest some funny group games or topics that don't involve teasing your friend. Explain to her that while you love her sense of humor, it should not be at the expense of your friend's feelings.
Another option is to include your girlfriend more in group activities rather than one - on - one interactions with your friend. This way, she'll be less likely to single out your friend for teasing. For instance, organize group outings or game nights where the focus is on the whole group rather than individual interactions.
Socially, if word gets out, it could lead to a lot of judgment from others. Family and other friends may view you all in a different light. And from a relationship stability perspective, it can completely disrupt the normal dynamic between you and your girlfriend. It might create an imbalance where one person feels left out or used, which is not a healthy foundation for any relationship.
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.
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.
One should firmly oppose and condemn this behavior. In a healthy society, we should uphold positive values. If the situation is serious, it may be necessary to report it to the appropriate authorities or institutions to ensure that ethical boundaries are maintained.
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.
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.