I am not condoning how she did this but I don't see the imminent danger or safety issue.
I think your real issue is being left out of the loop. Ye are separated.SShe can do what she wants ....... within reason. I don't think you need to or should react immediately. I'll let others who have more experience advise but that is my opinion. I do think you should reflect on when ye do talk as to what you will say.
I would act as if she is a neighbour. Ask how she is and move on as if it is none of your business. No big deal. But I would add that it is preferential to be aware of such events as it could impact on children
I am rushing here. That is just my initial reaction. I understand why you are unhappy. I hope you get the advice you seek.
R 25 years M 14 years S11 & S13 Working on it alone since Oct 2014 M in trouble a lot earlier (~2 years) Feb 2016. 1st R chat in a yr. Next R chat Aug'17 Still together