I’d also distinguish between life-changing on a one-time basis, life-changing at a pivotal moment, and life-changing on an on-going basis.
The OP’s friend got a rather small sum, but at a pivotal moment where that small sum set her up on a more successful path. But once she had gotten that new job, to credit everything that followed in her life to that free car seems wrong too. It was on her critical path, but perhaps she’d have gotten the job anyhow. Or perhaps she’d have screwed up that job somehow and ended up just as broke, albeit with a newish used car.
The McDs worker may well use the $50K simply to pad their budget by e.g. 10% for the next few years. And when it’s gone, it’s gone. Which 10% can be the difference between grinding high-stress poverty, skipping meals the day before every payday versus simply being “working poor” and frugally making ends meet reliably month in and month out. Barely. Is that life-changing? It’s sure day-to-day-experience-of-life changing, even though either way they’re still slinging burgers 10 years hence.
Or maybe they go go blow a bunch of it ASAP and then pad their monthly budget by less each month and/or for a shorter time. Or they may do something pivotal with it like pursue the higher education they knew they could never afford.
Which of those are life-changing? Good question.
And of course there’s what the other posters above talked about, money that’s big enough to alter your relationship to working at all, and/or materially alter your life circumstances.
Speaking as a 1-year retiree, it would take a lot of money e.g. >$10M to really alter how I live now. OTOH, a much smaller lump sum delivered 11 years ago might well have led me to retire 10 years sooner and therefore to have had a very different last 10 years.
My bottom line:
I think of “life changing money” as an amount that, regardless of your current economic station or your current phase of life, means that your life afterwards is set onto a trajectory you could never have achieved otherwise.
In one sense, the OP’s friend with the gift car simply went back to the middle class existence she had before some bad luck really knocked the wind out of her sails. So not life-changing, although certainly life-restoring.
Although as our own @Broomstick could attest, once the economic wind has been knocked out of you, climbing back up to where you had been before can be a very arduous and ultimately impossible task without some sort of windfall to carry / push you over the hump. No matter how hard you work and how carefully you budget, sometimes it just can’t be enough to overcome all the obstacles Fate has strewn in your path.
We’ve certainly had threads and threads about the homeless trap. Where the difference between making rent every month and being evicted comes down to $10, but once evicted they’re on a downward spiral from which recovery is borderline impossible. Would a single $20 bill be life changing at that critical moment. Yes, at least until the same crisis occurs next month. So maybe not really.