Back in the 1970's, my babysitter was a long-haired hippie. She always let us pee outside.

I guess it was easier than hustling all us kids back indoors, just so one of us could use the potty.
I've also, on occasion, seen people with young kids let them pee up against a tree.
And (my favourite story):
A friend of our family was driving us up north one winter, so we could attend my father-in-law's funeral. My children were about 4 and 6. He was speeding, and a cop pulled us over. My friend (lying, like a rug!) told the cop that he was just in a hurry to get to the next rest station because my little boy had to pee. The cop said, quite reasonably, "Just let him pee on the side of the road. It's fine!"
So, my poor wee boy (who didn't have to pee at all) was hustled out of the car and made to stand in a snowbank and told to pee. Of course he couldn't. My friend says, "He's shy!" The cop looks like he's not at all fooled, and is trying not to laugh.
Anyway, he was good guy and let us go with a warning about speeding.
(Which didn't work, because my friend got pulled over again with us in the back. This time he told the cop he was late for the funeral - which he wasn't. Honestly, those cops were WAY nicer than my friend deserved!)
Needs must, when it comes to young kids. Wet pavement is better than wet clothes. Some people are quite casual about little kids peeing in public.
I wouldn't feel bad for the little girl, but I would mention it to my neighbour, since she might not want to have urine soaking into her driveway. And if she has strong feelings about children peeing in public, then she will want to clarify that with her nanny.