If I was in the mc's shoes, depending on what happens, this is how I'd react:
1. Chad & Troy don't notice the mc:
I'd duck back out, and then just fucking tell Sage. I'd say something like:
mc: "Sage, you're not going to believe this, but I think it might explain a few things. Chad is gay.
Sage: "Fucking bullshit!"
mc: "No seriously, earlier today I walked in on him kissing Troy. There's your side bitch right there."
Sage: "Fuck, that really complicates things..."
mc: "Just forget him, your relationship is over already."
Sage: "No, I'm going to talk to him, he should have had the balls to tell me this himself!"
mc: "Do you want me to come with you?"
Sage: "No, I'll be fine, but you came through for me."
mc: "What do you mean?"
Sage: "You said you'd help me find Chad's side bitch, and you did."
2. Chad and/or Troy notice the mc:
I'd confront them and tell Chad he has to tell Sage:
Chad: "Oh fuck, it's not what you think!"
mc: "Chill man, who you wanna fuck is your business, but you owe it to Sage to let her know that this has been going on."
Troy: "Fuck off, this has nothing to do with you. You fucking tell anyone about this..."
mc: "Fuck off Troy, I already put you down once. I'm not gonna blab this to everyone, but Chad, you need to do the right thing; either you tell her or I tell her, but she deserves to know."
Chad: "Fuck, you're right. Fuck!"
mc: "Just one thing I gotta know, were those panties for you Troy?
"
Troy: "Oh fuck, my life is over..."
Either way, Sage needs to know. What Chad did to her was very uncool and he needs to face up to it. Who knows, there could be more to his story.