As the parent of a child with ADHD who was kicked out of preschool for behavior like you've described (a parent complained and got him kicked out), I can offer some perspective.
It may seem like she's not disciplining, but fact is she's probably doing everything in her power just to hold herself together and not make a scene. When you have a child doing mortifying behavior, you're under the spotlight and it can be hard to know what to do in public. If you're too firm, people glare at you, if you're too soft in how you handle things, same thing. This doesn't mean she's not doing everything possible to correct his behavior at home, but public can be a much tougher beast.
Here's what I would say in response,
"I feel I need to be honest with you. While I can see that K is really a sweet boy, his more violent tendencies have made my daughters a bit frightened of him. Have you discussed his challenges with a doctor yet? I know sometimes kids can have medical conditions that cause these types of challenges. He seems like a genuinely good-hearted boy and not a bully, so that's the only reason I bring this up. I wish you all the best, your name."
To me, I would have appreciated the honesty, while also appreciating that someone actually saw the good in my child, too.
I think a short response to her email should be fine. And I also wouldn't be shocked to see K kicked out in the near future, so any form of kindness you can give to the mom will mean a lot.