I have a friend group of four in my class (2 boys including me and 2 girls) and one of the girl is my gf and my friends also know that.The problem is that my friend wants to get closer with my gf and I can't confront him without any evidence and even my gf can't say anything. So what should I do in this situation?

She talks with many boys of my class in a casual way but we don't get to talk with each other idk why?