How hurt was her child? If it was a minor bump, it sounds like she overreacted, and was way OTT. I wouldn't shout at another child unless they were deliberately hurting one of mine.
She might also have been wound up if she thought that you weren't doing anything to make your ds apologise - and she may have had no way of knowing about his autism & ADHD. No excuse for what sounds like aggressive behaviour on her part actually, but it might be the reason why she overreacted if she thought you weren't doing anything about it.
My ds also has autism, and would never hurt somebody deliberately. If he knocked into someone I also wouldn't shout at him, though I would try and talk to him about saying sorry - but in situations like this there isn't always the time right then and there. It doesn't occur to most people at all that sometimes there might be other things going on.
But I think it can make us as parents a bit over-sensitive though sometimes. Sadly we often have to suck it up that other parents think our children are rude/naughty etc - if others are reasonable there's sometimes an opportunity to explain it, but it sounds like this woman wouldn't have been approachable to do that.
I had a woman shout at me in a supermarket once that I was "blocking her way arguing with my son" when I was trying to calm him down about choosing a sandwich. If she had been calmer I would've explained to her about his autism, but she was so bloody rude it got my back up instantly too.
Sorry, but I do think you were BU to report to the Head though.