(Possibly NSFW?)
So, the backstory for this is that my best friend of a few years recently became really good friends with this guy. He's a bit older than us and he's been dating this girl who's another friend of ours since last summer. They're very in love and they're very different people. He's always been known as sort of a manwhore and she's a very conservative Christian girl. He began going to church for her and changing his demeanor completely. He called down and began to seem much more mature.

He was very aware of the fact that she was saving herself for marriage before they began dating and knew exactly what he was getting himself into. Recently, he started texting my best friend and flirting more than he ever had. He's always been one to make dirty jokes and flirt with even his male friends, but this was very different than the usual jokes. He sent her a list of everything he found attractive about her and at some point he mentioned how pent up he was because his girlfriend won't do anything more than kiss him.

Eventually he brought up the idea of sending each other pictures and messing around. She told me all of this very soon after it happened and seemed to feel no regret about any of it. She wanted to continue everything with him and apparently fell in love with him because of this. He sent her random dick pics and said "Maybe one day, baby," in response to her wanting to have actual sex with him if they ever "accidentally" ended up alone.

Fast forward to a few days later and he texted her that he regretted the whole thing and they could never do it again. He said he loved his girlfriend and didn't want to lose her. She was extremely upset about this because she believed he loved her back and wasn't happy with who he was dating. I thought she was being insane because he was finally coming to his senses about how what they did was wrong and she still didn't see it. I didn't tell her any of this.

That day, I was talking to a very close friend of mine (I've known him for 9 years) and was ranting about how I wasn't sure if I should talk to the guy, who I'm sort of close with, about how he needs to come clean or tell his girlfriend myself. The next day I found out when my best friend began screaming at me that my close friend had accidentally let the whole situation slip to someone who told the guy's girlfriend himself.

The girlfriend is a very nice girl, she hasn't even broken up with her boyfriend because she believes he won't do it again, but she's obviously destroyed by the whole situation. She broke down and told a bunch of people that I was the one who let it out and now there's a whole hoard of friends of that couple and of my best friend who are pissed at me for telling someone.

My best friend thinks this is the worst thing that's ever happened to her because the guy won't talk to her anymore and she "just wants to be friends" now. She thinks the only thing she did wrong was telling me, not helping a guy cheat on his girlfriend.

AITA for getting involved?

TL;DR: I told someone about a mutual friend cheating on his girlfriend while trying to figure how to help, and now everyone we know has heard about it.