I think that sounds more like you projecting your opinion onto Riona. Riona knows that Quinn proposed to 'use Sage" to get close to the Preps so that she could find more customers. Riona then lies to Heather, telling her that going to the Prep party was Sage's idea, when she knows for a fact that it was Quinn's scheme. Nothing justifies that lie, not her desperation for money, not defending Quinn, she could have just remained silent.
What I don't get is how you can infer any affection or loyalty to Sage from Riona's actions. Her lies clearly put Sage in a bad light, when in fact all that Sage did was to cancel a social engagement with the DIK's. A social engagement, not a paid commitment to act as hostesses at the Dik party, as is clearly demonstrated in the DIK's post Prep party conversation.
Quinn planned to (and subsequently did) "use" Sage, she didn't propose undermining, sacrificing, or betraying Sage. So I'm not sure how that's relevant. Sage was used as cover for Quinn's activities, which is entirely consistent with viewing her as acceptable collateral damage.
But Riona also says "We...shouldn't talk like that" when Heather openly questions Sage's judgement. She immediately blames Quinn for ditching the DIKs as soon as the MC agrees to keep it a secret. And in Episode 6 she asks the MC to take care of Sage. Those are also consistent with viewing Sage as an acceptable casualty, but not with deliberately undermining her.
Also, Riona did keep quiet the first two times Heather asked about why the HOTs needed to be at the prep's crappy party. Riona only mentioned Sage when Heather kept challenging the logic. The goal was clearly to discourage Heather's line of inquiry by appealing to Sage's authority, not to make Heatehr question even harder.
Even the lie is pretty minor: it wasn't Sage's
idea, but it was still her call to arrange the party. All the financial mumbo-jumbo Riona parroted
was the reason Sage did it. Since Heather is part of Quinn's lifestyle, it's odd that Heather doesn't already know that. But that is clearly because Quinn is keeping Heather in the dark, this isn't some separate gambit by Riona to undermine Sage on her own.
So I'm not sure why you seem convinced Riona must be trying to do so. Is there anything you can point to that is consistent with trying to undermine Sage but inconsistent with using Sage as a shield for Quinn's activity?