Early on ds1 said hi to her and she didn't reply
Here's where the little girl made her opinion on engaging with your child clear.
The girl moved and ds only being 3 followed her and they both tried to look through a gap again.
Here's where she showed she wasn't comfortable with your child being near her, and you excused his behaviour in spite of her lack of comfort with the situation. This was the time to intervene. He should not have followed her to the new spot, and if he did, you should have stopped him. He is 3, you are not.
the girl hasn't reacted at all, no crying saying ow or even moving away.
So what? She could still have been bitten. The severity of the bite is not the focus, and her reaction has no input into how the situation is handled.
I told the woman she should have told me if she saw ds doing something she didn't like and I would have sorted it.
No she shouldn't. Not when her child was involved. You had already demonstrated that your child was the priority not hers. You had ignored the girl's right to personal space with the excuse that your child was 3. If my child is hurt, I deal with it. If another parent doesn't like it, tough shit.
You missed the opportunity to deal with this alone prior to the incident. You ignored the girl's responses that she didn't want to engage or be near your son, and still you allowed him into her space, and to follow her when she tried to get away from him. Had you respected her right to not have to engage with your son to begin with, this situation would never have arisen. He would not have been near her, and there would have been no question as to whether or not he had bitten her, or whether or not shouting was appropriate.