Years ago I was working in retail and became very, very close with one of my co-workers. I'll call her A. I confided a lot in her about very personal stuff and came to trust her completely. Some of her best friends (all guys) would stop by on our shift together and hang out with us for a bit. One of them, J, was pretty into me and would often tell A this - I was engaged at the time so even though the attraction was mutual, I obviously didn't pursue anything. I actually tried getting A and J together but she would always tell me that he was an idiot and she wasn't attracted to him and that she could never, as he was like a brother to her.
Fast forward maybe 4 years later, I'm at a new job but me and A still keep in touch. I broke off my engagement and asked her about J since I hadn't seen him in a while. She told J I was interested and we all hung out one night and I gave him my number. Basically we fell in love pretty hard and fast. About 8 months into our relationship, he ended things with me because he had a lot of mental and emotional issues when it comes to relationships (stemming from his childhood, blah blah blah) and I was beyond devastated and heartbroken, and he was too. It was a super angsty break up. Of course, because she was one of my best friends (and one of his too) I looked to her for comfort and a shoulder to cry on. She would tell me what a piece of shit he was and how he didn't deserve me, she claimed she would try and talk to him about how he fucked up and how I could do better.
A month later, me and J got back together and she lost her mind. While we were sorting things out, we discovered that she was telling him the same things she was telling me, that he deserves better than me etc, basically playing both sides of the fence. She also told their group of friends all my personal business and I didn't find out until then that she was an infamous backstabber and a huge POS in general, because I was so trusting I would never think she would ever betray me. So pretty much during the most heartbroken and hurt I've ever been in my life, this bitch was doing more damage then smiling in my face and wiping my tears.
Thankfully J never listened to her and knew better and realized he fucked up and things have been amazing between us since - we are getting married next year. As for A, a lot has come out since about what a huge piece of shit she is (apparently when we got engaged she went on a rampage and said some really nasty mean stuff) but we cut her out of our lives and haven't spoken to her in about a year. She recently requested me on Snapchat and saw J the other day while he was out and about and said "hi!" He mumbled hello and kept it moving.
So needless to say, even if she came to my door groveling and begging for our forgiveness, I would spit in her face and HELL NO I would never trust her again. To this day she is the only person who angers me enough to the point of a black out.