IMO the WAS is often in a period of self centeredness or self focus. They often perceive even the most pedestrian comment as an accusation or persecution. This is why it is often best to withdraw as much as possible from the equation.

I realize this is impossible when there are children involved. They are affected by far more than we may be willing to acknowledge. Shielding them from everything is impossible. The best we can do is being supportive and minimize the negative impact from our actions. Own your actions, and allow her to own hers.

A couple of things I have learned over the years are to pick your battles and fight only those battles you believe you can win. It is often counterproductive for the LBS to point out where the WAS failed. It is better if the WAS feels the sting of failure on their own

You had taken care of your son getting him to school. You showed him support and helped him move past his initial disappointment, all good things.

Where it fell apart was when you informed her. At that point you could have been perceived as rescuing him by persecuting her. She may already have her defenses up and perceive any communication from you as an attack. In that instance she is more likely to react with a counter attack of her own.

Others may disagree and you may not like this.
Support you son. Permit her to perceive her own short comings on her own.

BITS
Me 55, ACK, when did that happen? Doesn't feel like 55
D 30
S 27

You create your own universe as you go along - Winston Churchill