I’m very conflicted

Backstory- So please no judgment. I had sex with my best friends boyfriends brother. She knows and her boyfriend knows and they both are fine with it. Both of us knew it would be a friends with benefits type of deal. I was okay with that because I’ve never done anything like that before and I wanted to see what it was like. I lost my virginity to this guy. I have no emotional attachment to this guy (surprisingly). After having sex and giving him head a couple of times I realized it wasn’t for me. I told him that and it was good we were just gonna remain friends. A few days later he hits me up and asks for nudes and I was like “like I said before it’s not for me to mess around with people that I’m not dating” and he said sorry won’t happen again.

What I’m actually conflicted about- So I asked from the very beginning to be straight up with me. He went along with it and he had supposedly just broken up with his girlfriend a few weeks prior. I’ve asked him again multiple times and right before we had sex if he was still messing round with his girlfriend and he said no. After he asked me for the nudes I did some researching and found her online.... they have been in a relationship since 2018. I feel so sick to my stomach. I’m disgusted by the disrespect from him and I can’t believe that he would do that to her. I feel like my heart is in my stomach because i know what it feels like to be in her position. I feel so terrible for her. I’m just so disgusted and appalled by him and I just don’t know what to do.