Fwb when he has a gf??

So my best friend of 10 years and I have been close (sexually) lately. I know he likes me. He’s said so. And even wanted to be friends with benefits. But he has a gf. His girlfriend used to be my friend. But she’s bisexual (so am I) and summer 2018 she asked me out and I turned her down bc I knew it wouldn’t work out with us both having mental health issues. Then she goes to another girl. Rejected. Then she goes to my best friend (of 10 years) and he says yes. And why did she ask him out? Because I told her I was going to. So instead of telling me that she asked him out and stuff, she told me he was just gonna reject me and to not bother asking. Well here we are, almost summer 2019. Me and him are closer than ever. And hes had to lie so much to her. She doesn’t want me anywhere near him. She never has. She doesn’t even want him texting me. So he’s told her that I’m dead to him. But he said that so she’d stop worrying. But me and him talk just as much if not more than him and her. And now we’ve been. Sexually close. Mostly over text. But we’ve played around a couple times irl. I’m live within walking distance from his house. And he knows it’s wrong. I know it’s wrong. In my eyes she’s just trapping him. And he’s said that she acts like she cares, and that enough for him to stay. Take in mind she’s been in juvy, drinks and does drugs (says she doesn’t now but idk) and even says she’s stopped cutting. He doesn’t have an Instagram. She has a video where there’s clearly fresh cuts on her thigh. I’m not sure where I’m going with this. I guess I just needed to get it out..