My BFF’s Boyfriend

My best friend has been dating this guy for almost four years. Four freaking years. Him and I don’t click but we’re polite to each other and respect each other’s role in her life. But I just know that she’s settling. And being around her is so uncomfortable because she’s become this person that’s perpetually irritated and the second you broach a subject (like him) she gets super defensive and swears she loves him. But when she talks about their relationship she sounds so like “this is it, this is what relationships are.” I don’t think he’s a bad guy necessarily but I can just tell he’s not the one for her. And everyone else in our circle of friends, even her family who I’m close with feel the same way. I don’t even want to be around her anymore because I don’t want to be around him. Or even if it’s just us two I just don’t know how to be around her because the reality that she’s considering spending her life with him and starting a family just, I don’t know, like that’s going to be the guy she comes home to and has kids with and it’s just like, I don’t like who she is with him. I just don’t know what to do. I fear that we’ll lose our friendship