r/AskWomen or r/AskMen have more discussions about fwb stuff depending on who you're trying to get advice from.

If you think you're in control, it shows me how naive you are. Take it from somebody that comes from a family of cheaters. Its the main reason I dont cheat. The side thing always thinks they have power and stuff, but the reality is he has a girlfriend and you aren't going to change that unless you blow his world up and tell the girl.

Also, I get you don't care about the other girl because it doesn't affect you, but you need to learn some empathy because being with cheaters is going to get you cheated on. Even if he did break up with the girl and you ended up together, you'll end up being the one cheated on by some girl that doesn't care about you. What goes around comes around so good luck to you.