It has nothing to do with who he cheated with. My personal opinion is that it's not fine to out someone, and if MC tells Sage what he saw, he's effectively outing Chad, which, again, I personally think is fucked up, regardless of what he did/didn't do to Sage.
I mean, if that's how she takes it, then fuck it. But that's 100000% not MC's secret to tell, in my opinion, regardless of the circumstances. At most, I'd be okay with MC talking to Chad privately and trying to encourage him to tell the truth.
Tbh, in this case, I don't really think it's about what's best for Sage. Obviously, what's best for Sage is to be aware that Chad didn't act that way because of her specifically, but because he's attracted to men, but is that what's best for Chad? Chad's secret is a very big one that would likely shake his entire world up if it came out, especially if it came out in a way that he wasn't able to control. Sage thinking her sisters fucked her man is... super minor in comparison, and if Sage ever truly loved Chad, then I'd hope that she wouldn't want him to be outed against his wishes either, no matter the circumstances. Again, this is just my personal opinion.