I posted something recently too about "mean" girls in school.
It's amazing to me how mean kids are - even at an early age. What's even more surprising to me is how it seems so many parents just chalk it up to "kids will be kids". You don't say what this mom did or said, only that she seemed embarrassed and somewhat sorry.
I'll tell you, I feel like a drill sergeant the way I MAKE my kids apologize, make them look someone in the eye and open their mouthes (not mumble) or remove them from any situation where I feel they are not behaving appropriately. Even in my own family my nephews can behave like little animals without their mothers so much as getting out of their chairs to correct them!
I feel your pain. Although I completely get why a little 17 month old might push people - it's how the adults around them or who are responsible for them act (or don't) that really floors me.
I'll tell you from experience. There will be mean kids in EVERY class. Kids who think: "If I see it, it's mine. If you have it and I want it, it's mine. And if you don't give it to me, I'll yell 'YOU'RE MEAN!' or push you, hit you, etc''.
The next time you go to MDO, DO NOT sit by that child and mother. Maybe get there a few minutes early and "make friends" with another mom and tot. Then your daughter will have someone to play with.
And for what it's worth. If/when my child has ever been hit, pushed, etc. I have no qualms about telling that other kid, "HEY! That's NOT nice." I don't wait for the mother or another teacher to see it. Chances are "those moms" who aren't on top of their kids don't really care or won't do much about it anyway. Now, that said, all kids are going to test limits and do naughty things. It sounds like that other little girl was doing it a lot that day, which signals to me that it isn't her first time...which then leads to the question of, where was her mother when all of this was going on???
Best wishes.