If you are wanting to continue the relationship, do leave the present, and leave small card. I would be very thoughtful in my wording, don't make assumptions, and try to leave the door open. (I'm suggesting a card because email tends to invite an immediate response and you want her to be thoughtful too.)
"I will say that I think the whole thing has been handled poorly and I valued our friendship more than she does evidently...... "
You are making an assumption regarding what she does/doesn't value. She might feel very upset about this: she doesn't know exactly how to 'solve' this problem and may feel her parenting skills/knowledge are being attacked. People who start to feel terrible as a parent or judged can be very hurt. Remember, at some point there was a discussion in which you asked her to change the subject so you wouldn't get upset. I'd take that as a blaming move, not a neutral one. You likely thought you'd just changed the subject and moved on. And you did get the last word by doing that. She might have been deeply offended that you took control of the conversation, no problem-solving was achieved, and that in order to momentarily keep the peace, she lost her 'voice' in the discussion.
I know, on my end, that it's hard to have a more docile child sometimes. My son gets dragged around by the arm at preschool; my husband noticed this and asked one of the teachers (who I trust implicitly), who replied that they were working with our son on asserting himself. It wasn't the answer my husband wanted to hear, but she had a point, at some time, our son is going to have to start to stand up for himself with his peers. It's true, we don't like to see our kids getting pulled around or being overwhelmed by other children, and I have had to learn not to step in and correct the more physical child, but to encourage the more submissive one, "You look upset because he's grabbing your arm. If you want him to stop, look at him and say "Please stop. I don't like that." We were role-playing a little bit in the car with our son earlier today, and it's *work*. He really has to be given a lot of coaching "use your strong voice, tell me so I can hear it" to be assertive.
I hope everything works out.