I have to say, I would not only NOT just demure and decline discussing the event, I'd go on the offensive. Don't just forward the email to dean, tell her you DEMAND a meeting with both the dean and her to find out why she feels she has a right to pull a student out of your instructional time without even the courtesy of asking you first. (I'll bet the dean probably has a very different understanding of her right to talk to students whenever she wants.) There's nothing impolite in being assertive about addressing wrongs.
I had a similar situation when I was teaching kindergarten. A fifth-grade teacher pulled me into the hall (while my kids were in the classroom) to discuss a parent of mine who had witnessed her son, who was in the other kindergarten, push a classmate of his while they were lined up outside before school started. So, an event I hadn't witnessed, involving no students of mine. Finally, I told her I would come talk to her when my kids went to art. Oh , no, that wouldn't work, she would be instructing students at that time. I looked at her, back to my students waiting for me, back to her - and suggested we talk during our only mutual prep time, before school the next day - in the principal's office. She backed down really quickly after that. She was very pushy, but I politely refused to be pushed around.
This was all a case of professional Darwinism, too - she was a first-year teacher at that school, without tenure. Add to that an ill-behaved son and a defensive streak, and shockingly she (and her son) were not asked back the next school year.
Oh, the making the student cry really angers me, too, but other posters have covered that. I just think a lot of teachers can be "too nice ," and I don't think this is just between her and the dean and the student - it affected you and your whole class, too. The angry email would've probably come from me first!
Edited to add: I can totally see not going to the Dean - as she suggested, you could "come to her" and read her the riot act for her presumption!