The phrase others used leapt to mind: "Cut her some slack." I believe (and I have a girl who's now 10) that five is really too young for true, intentional "mean girl" behaviors. Instead, she is (a) in that normal but annoying bossy five-year old stage, which both boys and girls can go through; (b) going through that stage alone, with zero parental correction, at least at the McDonald's -- remember, you have no idea what the rest of her life is like, and she may get plenty of attention from mom, or from dad in other settings.
She was attracted to you because you dared to correct her; did you notice how, instead of being pouty and turning her back on you or talking back when you corrected her (which an older mean kid would have done), she was attracted instead? That indicates to me that she's not a mean-to-the-core kid; she mostly just wants attention and yes, some boundaries.
The next time you go to McDonald's this is what I'd suggest: Go with the thought that if you see her, you will actually intend to interact with her a little more. On purpose. Say hello to her with a big smile. Have your son introduce himself to her and ask her name and say "I've seen you playing here before." Tell her "Have fun! This is a great play place, isn't it!" as they set off to play. They don't have to be forced to play together, of course. But if she seems to be getting bossy, go over to her and try something like this: "Hey (Sally), would you help (Billy) and me put these balls in the bin? They're spilled all over the place and I'd sure like your help putting them back." (i don't know much about these play places but I'm figuring here they have things like plastic ball pools...) Improvise. In other words, see if you can find some little responsibility for her to take on right there and then. That may make a huge difference. I think the dad, if he's as checked-out and lacking interaction as you say, will not mind; he ought to welcome it. You're not trying to "run" her, just interacting enough to help her calm down and be less bossy, which benefits your son too. Bossiness can be channeled into responsibility -- that's the positive channel for it at this age. Of course you don't have to do any of this, but it would help this kid if you do. While there's no way to know what the rest of her world is like, I wonder if dad's always this hands-off....