AITA for exposing my husbands best friend and breaking my best friends heart?

I (f34) recently got married, and my best friend (f34) was my bridesmaid. She had just gotten out of a 10-year relationship and engagement, which came on the heels of one of the worst years of her life. She’s been healing and trying to move on. My husband (m45) had his best friend (m45) as his best man during our wedding weekend. At our gathering in Big Sur, my best friend and my husband’s best friend hit it off. They didn’t hook up at the time, but once we were back in LA, they started spending a lot of time together.

It was like watching two people fall head over heels. They were giddy, always holding hands, and constantly texting. They acted like a couple whenever they were together. Even after my best friend returned to her home abroad, they kept in touch. He even met her in Austria for a trip after our wedding reception in September. But soon after, things started to unravel.

His behavior became inconsistent. For his birthday, my best friend sent him a heartfelt, personalized gift—a poem and a carefully chosen present—but he didn’t even acknowledge receiving it until she asked days later. Meanwhile, he made no effort to reciprocate or show her the same level of thoughtfulness. Over time, he started giving her hot-and-cold energy: one moment, making plans to meet every four weeks, the next, pulling away and acting distant.

In late December, she came back to LA and stayed with me for my birthday. He barely made an effort to see her during her two-week visit, citing excuses like his kids and the holidays. When they were together, he was cold and withdrawn. On New Year’s Eve, she ended up at his house but later said she regretted it, as it only added to her confusion and hurt.

For months, she’s been calling me, trying to make sense of his actions. She’s even tried to end things with him several times, but he always refuses. He’d say things like, “No, no, no, that’s not fair,” or promise to call her back to discuss it—then not follow through. Meanwhile, he’s had plenty of opportunities to walk away and end the situation, but he hasn’t.

Then, last week, I overheard a conversation between my husband and one of their mutual friends. The friend revealed that my husband’s best friend’s ex-girlfriend, whom he dated on and off for two years after his divorce, is back in the picture. This ex is someone he’s spoken poorly of in the past, so I assumed they were done for good. Learning this was a gut punch.

I’m furious. He has disrespected, mistreated, and strung along my best friend while seemingly rekindling things with his ex. I feel so betrayed, and I’ve completely lost respect for him. I would never have introduced my best friend to him if I had known this was who he is.

Now I’m torn. My husband wants me to keep this information to myself, saying it will only hurt her and doesn’t need to be shared. But I feel like this could give her the closure she’s been searching for and help her move on from this painful, confusing situation.

So, Reddit, AITA if I tell my best friend that my husband’s best friend has been seeing his ex while stringing her along?