I have a friend, let's call him "Bill". He has been one of my closest friends for many years. He has been dating this girl, let's call her "Natalie" for almost a year now. Natalie is also a very close friend to me, due to the fact that A) I knew her long before they were dating, B) she's very close friends with my girlfriend, and C) I spend a lot of time with the two of them.
But now I'm in a little bit of a weird situation. Natalie will frequently complain about Bill publicly to other people, even in front of him. From really small minor things, "His new facial hair is horrible. I can't wait for him to shave", to larger things, like "The way our relationship started was really awkward because he was too dumb to just ask me out". At first, this was just minorly annoying, but it's starting to concern me a little bit because it's becoming a more common occurrence. To be clear, Bill has never complained about this to me, so I don't know what his feelings on it are. But I imagine that if I was in his shoes, I would feel very embarrassed and hurt. I'd expect my significant other to have my side, rather than publicly mocking me.
I'd like to mention this to Natalie, and actually, recently my girlfriend has been asking for me advice because she wants to bring it up as well. But the thought of talking with her makes me (and my gf) extremely uncomfortable.
She might feel hurt or resent us for correcting her. Not that she's a petty person, but because I'd imagine being chastised by your close friends would be rather difficult to take. Or maybe she'd feel like she's done a bad job of respecting Bill, or that she's "a bad girlfriend", I don't know.
But the bigger reason I'm uncomfortable mentioning this to her is because I'm not sure if I'd be overstepping my bounds. I don't want to come across like I have it all figured out, and I'm going to step in and dish out the perfect advice to fix everything. Or that how they do their relationship is my business, or my problem to worry about. But they're both very close to me, and I want them to be happy.
This leads to my question:
Would it be out of line for me (or my gf) to bring up this issue with her? Is it really any of our business? And if it is something you'd recommend talking with her about,
What is the most tactful way for one of us to approach her? I really want to be careful to display an attitude of "I want to help you, and I think this would be beneficial for you in the long run" rather than "You're screwing up and you need to get your crap together". And how could I bring it up without potentially hurting Natalie's feelings?
I don't know if it's at all relevant to the question, but the 4 of us are all in our young 20's and we live in the USA.