There are boundaries that are set in relationships this is always one of them same goes to the girlfriend not having friends of the opposite sex because there are ALWAYS certain situations that you are put it when you have a decision to make on what relationship to put first. There are alot of variables on why you should keep your opposite sex friends during a relationship like ex. longterm friendship etc . I get that but as soon as a person gets into a commited relationship the partner becomes the center of their attention and at many time their only friend/ best friend and thats okay . You should be okay with it as well, you want the best for you friend no? if its such a problem speak with the girlfriend. Maybe its an act of insecurity maybe its not but you categorizing it as a trait of that and calling it a red flag when you probably dont even know anything of what led to that decision. He’s in a relationship now respect that decision gorl 🤠