Uh…maybe she didn’t have the money to get there.
When my ex and I first divorced, I was left homeless. Then I managed to get the kids and me an apartment, but I couldn’t afford a car so we bussed or walked everywhere for 4 years. My kids’ school was clear across town from where I worked; it was an hour long bus ride, and buses weren’t constant, so I had to be leaving when a bus was also leaving for things to work. If school had called about one of my children (in fact I’m sure at some point they probably did), even if it was my week, I had to direct them to my ex if I wanted a parent to be able to attend anything last minute or immediate.
Didn’t mean I didn’t want to be there.
Maybe you’re right, and she’s a shit mom who’s too busy smoking crack to come hold her son’s hand.
But, maybe she’s not.