To me, at the end of the day, the big issue would be: was my child actually, really, safe?
Not, "did I worry that he might not be safe?"
But "*Was* he safe?"
He was safe. He was perfectly safe. He was safely strapped into a safe car seat in the backseat of a safe car driven by a safe driver.
So while I might have been a bit worried, I would be done worrying or thinking about this.
If I thought the timing might lead this mom to do a similar thing (i.e., if the group really wasn't that late in leaving the place they'd gone), I'd say to the director that I'd like it if they explored the "picking up your kids" issue with other daycare parents in the future.
And then I'd be done.