Her job is already going away; McDonald's around me have already replaced cashiers with kiosks. Unionizing and requiring $15/hr + full benefits is only going to accelerate McDonald's automation plans. Is the answer to fight for $15/hr or is the answer to go back to school, apply to more jobs, and/or go into a trade? Honest question.
It's not like they can replace human personnel that easy. Ordering kiosks are the low-hanging fruit, and then comes a long, long stretch of nothing, because it turns out that developing McDonalds-compatible robots capable of preparing burgers and packing up meals is about as easy as developing actually-working autonomous vehicles. And about as expensive. It's probably easier to automate the managers away. Meaning the…
For argument's sake, let's assume that no jobs can be automated. The cost to the franchise owner to operate goes up, and in response they raise prices -- but at some prices they're going to really start losing business, so they also have to lower costs elsewhere or simply get lower profits. How does it all shake out? The fact is that it has to be paid by someone, and it'll likely mean fewer McDonalds are opened because it'll just be harder to make them profitable.
Maybe you're OK with that trade-off, but it IS a tradeoff that will happen, and it means fewer jobs.
Your analysis presumes that the business owners can and will absorb anything without reacting, as if more value can be extracted but there won't be a new equilibrium. I assume that the franchise owners aren't making the kinds of profits where that is true, I assume they're mostly bumping along on fairly low margins.