My best friend is annoying me

IDK how to deal with this situation. My best friend is a single mom like myself but she hangs out with her babies dad ALL the time; even without their baby. He is in love with her and she loves someone else. She is ALWAYS stringing him along, going back and forth between trying with him and the guy she truly loves, always talks to me about how he’s basically the back up plan and how she doesn’t want him to be with anyone else either. It disgusts me and I’m having a hard time respecting her therefore a hard time tolerating her. Don’t say it’s not my business because it is. We’re best friends and I have to hear about this all the time and see it all the time. I think her baby’s dad is just stupid honestly. I have no idea why any man or anyone for the fact would tolerate being treated like this! I just don’t get it. I don’t know how to tell her to stop being this way. Without her getting so offended about it. She’ll probably say it’s because I’m bitter about my own baby’s dad even though we have a very healthy coparenting relationship.