It's just that this situation, we had an agreement and he told me he would talk with her and wanted the same agreement, then just said something along the lines of "conversation went really well. She's in agreement." Then, months later, she mentions to me that she essentially disregarded it because she thought he would come around.
If I'm understanding correctly, he kept agreements but meta didn't. She told you that she didn't see those agreements as permanent and now you want to white knight your shared partner from her because you believe you know something he doesn't because she spoke with you about this.
This may be because those agreements sounded more like training wheels to her and she's figured that he (and you by extension) would grow out of them. But neither of you have. Or it's because she doesn't want to have that agreement anymore because she finds it too controlling.
If anything, and if I'm right that she did confide in you, how about saying to her that that is a conversation she needs to have with him? Why wasn't that option 1?
FYI, when I was introduced to my now husband's female then bff I liked her and confided in her and she went straight to him without giving me a chance to. She has slowly drifted out of our lives entirely. Good riddance.