AITA for exposing my friend’s secret, even though she asked me not to?

So, I (27F) have a friend, Sarah (26F), who I’ve known since college. We were roommates, then we became best friends. Over the years, we’ve shared everything—our successes, our struggles, and of course, our secrets. I’ve always had her back, and she’s always had mine. Or at least, that’s what I thought.

Last month, Sarah started dating this guy, Mark (30M). At first, everything seemed fine. She was excited, and I could tell she really liked him. They were always texting, and she’d often gush about how amazing he was. But there was something weird happening. Sarah started becoming more distant with me. We stopped hanging out like we used to, and she was canceling plans last minute. When we did meet up, she’d always be on her phone, texting Mark. It felt like I was losing my best friend to this new guy, and I started feeling left out.

But then, things took a turn. One evening, we were at dinner when Sarah broke down and told me that Mark had a past—a past she hadn’t told me about. Apparently, he was still in touch with his ex-girlfriend, and they were, in her words, "still trying to figure things out." Sarah admitted she knew about it but didn’t know what to do. She was confused and hurt, but she was willing to overlook it because she was afraid of losing Mark.

She made me promise not to say anything about this to anyone, especially to other friends or family. She told me she was going to talk to him about it soon, but she wasn’t sure how. I agreed, but honestly, I was feeling uncomfortable about the situation. I wanted to be supportive, but I also didn’t think it was right to let her ignore such a huge red flag.

The next weekend, we had a small get-together at my place with a few of our friends, and Mark came along. The whole time, he was acting distant and weird. He barely spoke to anyone, kept checking his phone, and whenever Sarah wasn’t around, he was chatting privately with one of his friends. I couldn’t ignore it anymore—something was off.

That night, after everyone left, I took Sarah aside and told her I couldn’t stand by and watch her settle for someone who clearly didn’t value her. I said she deserved better than someone who was still hung up on his ex. She told me I was being harsh and that I didn’t understand her situation. That’s when I did something I still regret—I told her that Mark was still in contact with his ex and that I thought she should think long and hard about whether she wanted to continue dating him. I wasn’t trying to expose her secret, but I couldn’t let her keep ignoring the red flags.

She was furious. She said I had broken her trust and betrayed her. She told me that I had no right to interfere in her relationship, and she stormed out of my apartment. We haven’t spoken since, and mutual friends have started picking sides. Some think I did the right thing, that I was just looking out for her. Others say I overstepped and should have respected her wishes to handle the situation on her own.

Now, I’m left wondering if I made the right choice. AITA for exposing her secret and trying to protect her, or should I have kept my promise and stayed out of it?