Last year, I had sex with a guy, only to find out that he had a girlfriend. I honestly felt terrible because he continued to go on dates with her since, and she had no idea what happened. I recently found her social media and saw that she had posted a picture of them together.
I really want to come clean and tell her what happened between her boyfriend and me, but I am scared of what is going to happen next. I need advice on whether or not I should tell her, and I also need advice on what I would say if I told her.
LemonVibe 
This won’t help her. This is to help you. She might believe you and break up with him or she might go stalker psycho on you and try to break your face. Move on and warn other girls about him that know him, but leave her out of it. You don’t need that kind of drama in your life.
It’s a tough one. Sadly, women tend to blame the other woman rather than the men. Why? Because they are so silly in love with these men that they are waiting to hear what they want to hear, which is usually for their men to deny it. On the same note, if my husband slept with another woman, I wouldn’t want to walk around blissfully unaware; I would want to know.
That decision lies on you. You hold the cards in this situation. That girl deserves better than a cheating asshole for a boyfriend. There’s 110% he will do it again with a different girl if you stop sleeping with him. Eventually, he will get caught. They always do. But if you make sure you don’t become the “side chick” for this man, then maybe it’s best to let the drama go and just move forward. He will slip up in a bad way eventually.
Years ago, I unknowingly slept with a married man. He became stupid clingy right away and I had to lie to him and tell him I was going out of the country for business to get him to leave me alone. At that point I still didn’t know he was married. It was a few years later when I saw him on social media that I realized he was married then, and was married when we slept together. I sent a message to his wife and let her know what happened, when it happened, and offered to give her the e-mails that I had from him. She never responded, and blocked me on Facebook immediately. Another year went by and he e-mailed me and told me I had ruined his life. I hope I did. He didn’t deserve that marriage. But his wife? She never said thank you, she never acknowledged that I did anything.
A few months ago some woman from my husbands past sent my husband a facebook message, begging for him to answer the phone when she called him. He told her no. She kept on and kept on. I finally sent the screen shots to her husband and told him to put a damn leash on his wife. He thanked me for the information. She blocked me on Facebook, but not my husband. So, it can go either way, really.
It’s been a year. Let it go. Plus need more facts to make informed decision. Not saying what he did was right, in fact once a cheater always a cheater. However, without all the facts of his relationship with his girlfriend at the time, hard to make informed decision.
I wonder if you really just want to hurt him for betraying you.