This is a very common problem. This is how the story goes Boy falls in love with girl, girl falls in love with boy. Best friend falls in love with girl. And then the story can take a few paths and the path is decided by you.
If you are the more dominant leader of the relationship, you can break of all ties of your girlfriend with your best friend. You tell her you don't want her to see him anymore. But this is usually a bad path to take because this will most likely lead to a relationship of hard and sad feelings and probably the end of your relationship.
If you want to try something a little out of the usuall sit down with both of them and understand their relationship and from your understanding make a decision of whether to end your relationship with her or to keep it going.
If you don't like the girl and don't see things working out you might as well end this relationship to avoid conflict with your best friend. BUT be sure you have your facts straight cuz you don't want to prematurely end your relationship.
There are many other ways and its up to you to decide but if it was up to me I would ask them individually of what to do.
Hope this was of help
Elite Advice Specialist
