My boyfriends friends have no boundadies

So basically me and my boyfriend have been together for 18 months and he recently started talking to this one guy he was best friends with for YEARS, that best friend has a girlfriend and my boyfriend has a little bit of an issue where he over shares our issues with them sometimes and they never met me keep that in mind.. but to have that negative view on me from my boyfriend telling them stuff, we talked about it and he doesn’t do it anymore, but today his best friends girlfriend decided to message him and tell him that I’m no good for him and that I’m incredibly toxic and manipulative? That’s news to me! I didn’t even know that about myself lol. My boyfriend and I have had our fair shares of issues like any other couple but we have gotten better and we’re better than ever it just really bothered me how she decided to tell him one on one, she said I was immature and I act like a 16 year old? I’m 21 and she’s 20. I just don’t think it’s her place to be making comments about that especially when the stuff she’s heard is biased and one sided. I just don’t think it’s fair for me for them to have that assumption of me when they don’t know me and they’re thinking I’m this horrible person when I’m not. I just don’t think it’s okay for them to be over stepping into our relationship, how do I approach this???