I would honestly let it go.
If your dd doesn't count her dd as one of her closest friends, then I would just accept it and move on.
If the 2 girls were best friends and your dd wasn't invited then you might have a reason to think (not say to the parent) that there was a personal reason that she wasn't invited.
Some people, so I have recently read on mumsnet do not invite children to their DCs parties if they don't have "proper parties" of their own, I can't quite get my head around that one either tbh, maybe that's why she wasn't invited?
In fact, thinking about it, I can't really understand all the angst about childrens parties.
You have a party for your DC or not, whatever suits your circumstances
You let your child invite whoever the fuck they want without turning it into a great big drama of epic proportions
Your DC gets invited to some parties and not to others.
That's it. If people just did that, there would be none of this. But it's all such a drama, the poor DCs don't even get a say in who they invite because mummy is standing there with a checklist saying "oh no you can't invite x, he never has a party and you can't invite y because he didnt invite you but you can invite z because he invited you last year"
LET IT GO.