She shouted at my 2 year old. Really really shouted at her, probably more like screamed at her, for pushing a swing chair too hard with her son sitting on it (who as it seems was loving it).
So loudly that the entire garden stopped what they were doing, sudden silence. So loudly that my legs went to jelly.
I was so stunned/angry/overwhelmed I walked off. DD apparently went silent, face crestfallen and followed me. She came up to me, put her arms around my legs and sobbed.
DD told her that we do not speak to her like that.
She apologised.
I am still upset.