I don't see the two as mutually exclusive: if something bad happens to Riona because of Quinn's ambition, that can instigate Quinn's crisis regardless of what exactly happens. We don't know why Quinn needs the money, but she is obviously obsessed with getting it. It will take something major for Quinn to renounce her empire. We know that peddling coke wasn't enough. A friend (assuming she sees Riona that way) suffering for that decision might do it, but only if Quinn recognizes her culpability.
Given what we know of Quinn, if Riona ODs on coke she stole from Quinn, I think Quinn would be far more likely to blame the whole thing on Riona's stupidity and thus rationalize continuing on her self-destructive path. After all, she's been disregarding Riona's advice for a while now. If she's confronted with a glaring example of Riona exercising extremely bad judgement, will that really cause Quinn to take her earlier advice to heart? I say no.
If, on the other hand, Riona is badly injured while grudgingly following Quinn's orders, that might force Quinn to recognize the damage she has done - especially if Quinn continued to belittle Riona's concerns and browbeat her into line. It would, at the very least, be much harder to twist Riona's fate into proof Quinn was right all along. In this case, not only would Quinn would be all alone to face the music, but she'd have to face it knowing her judgement was inferior to Riona's.
That sounds like a proper crisis to me. As an added bonus, now that the coke wouldn't be the instigator of Quinn's moment of crisis, it could be a physical representation of that crisis. Once she hits rock bottom, will she reject the coke and start climbing up? Or will she embrace the coke and, as Tommy put it, get comfortable at the bottom?
Thus it's the perfect opportunity for our off-white knight MC to ride up and help Quinn resolve the crisis: if the coke is a manifestation of the crisis, then convincing Quinn not to use it is a manifestation of giving her a hand on her first step out of the hole she's dug for herself. If the coke's already been snorted by Riona, then what form will the MC's help take in the alternate moment of crisis? Sure, he can give her a big speech or a good boning, but what will symbolize why THIS time is different than the previous times?
I suppose she could always do the 8ball and then go back for more (giving the MC a chance to belatedly intervene), but that's a muddier line in the sand. Or maybe we can find a suitable metaphor in whatever Quinn 'needs' the money for. But until then I think the coke is the best fit for the job, so that's what I think will happen (at least potentially - the ability to influence Quinn might be complicated).