You're friends with A's mom, but the apple may not have fallen far from the tree in this case.
At the amusement park, it had to be clear your DD was being excluded when the new girl joined them and your DD was the one who had to ride alone. Yet Girl A's mom said not a word. No "Girls, let's take turns riding together." When I think back to when DD was little and she was friends with various girls and I wound up making friends with their moms, any of us would have spoken up and corrected that situation. None of us would have let the exclusion continue. A gentle suggestion of rider rotation could have taken care of it, but she didn't speak up.
At the playground, despite your DD's obvious distress, A's mom never said a word. She could have easily said to the girls, "There's plenty of room in that tunnel for all three of you," and with "the look" to her DD, that should have ended the matter. Again, I cannot think of one of the moms of DD's former playmates who wouldn't have done this. It's that simple.
To be blunt, she doesn't have an issue with her DD excluding your daughter. It's that simple. Why continue this friendship, be it between the children or the moms? Her DD has no problem being mean to yours and the mom has no problem allowing it. Quite frankly, I'd find a better class of friend. One more time, it appears the apple didn't fall far from the tree and you need to recognize that. Maybe Mom A not correcting her DD is actually her way of excluding YOU.
