would it be wrong to spill this secret?

So I've been FWB with my guy friend for a few months now. He's very secretive about us, we're in the same friend group so we pretty much sneak around. We're not together so I guess makes sense he doesn't want people to know we sleep with each other? Idk? But I'm close wit one of our mutual friends. Pretty much been lying to him for months and I wanna tell the truth but I know FWB wouldn't approve. Would i be wrong to tell?