Does that make her treatment of my dd acceptable? To excuse it or at least explain it?
Well, it might explain it but it doesn't make it acceptable.
My concern would be the impact it has on her longer term but, at this age, I was also telling mine that I couldn't get involved and she had to make these choices for herself. Then, again, in the grand scheme of things, they are still young and it's difficult for them to know how to deal with it.
When I was in 6th form, I was in a similar situation. One day, she just pushed too far and I confronted her and told her that she was a bitch and this was why nobody liked her. It wasn't true because people did like her! But I was also only 17 🤷🏻♀️
She ran off to the toilets crying, I felt terribly guilty but was also still angry and left it. She never did or said anything again, we became good friends and she's now one of the few people from school I keep in touch with.
I know what triggered it (I knew at the time too) and her unkind comments to me were just a way of making herself feel better and more in control but that doesn't make it OK either.
Your daughter is allowed to stand up for herself.