I acted similarly in a long-term relationship with my exboyfriend of about 3.5 years. We got along great and we understood each other's quirks and accepted each other's raw self. The problem for me was there was no spark. He never wanted to go out or have sex, when we did things together we didn't joke or talk or laugh anymore. We kind of started feeling like roommates. I ended up talking a bunch with friends (male friends) with zero intention of anything unfaithful -- and I never was unfaithful. I ended up having the laughs and conversations I with them that I used to have with my ex. I found myself wanting to hang out with them and not my ex because he was just so.. Boring?

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I'm not saying this is your situation, but you mentioned that there's been an emotional disconnect. It seems like your deep talk together may have brought that back for her and she feels like she can invest in you again. It is possible that both your relationship and the married guy's was similar and they were just able to talk and connect based on common issues. That's what my best guess would be. I tended to spend a lot of time venting about my frustrations and connecting with my friends who had similar experiences. It was pretty lame of her to stay out so late, but if she's anything like me then she was probably hanging out late talking about relationship stuff with her friend. Seriously, the number of men who I've been able to get relationship feedback from.. It has always helped me understand my partners so much better. The reason my ex is my ex is because we had conversations about my concerns and then he would put zero effort into even meeting me halfway. He was checked-out. It seems like you definitely aren't, which is why she feels closer to you again, I'm sure. :)

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tldr; Her and her friend could have very well just been able to vent to each other about their relationship issues and been able to connect as friends that way. Since talking maybe she doesn't feel so much need to vent because your relationship is better.