I'd have called the cops or park ranger and reported her for animal abuse. For someone who has an abused shelter dog you'd think she'd have behaved better.
This is the second negative dog park post I've read here today. Glad I don't have to rely on one for a place to exercise my girl. I have been advised by my trainer to never ever go to a dog park for these very reasons. Once off leash you have no control over something that can happen very quickly. Why was she at a dog park with a fearful dog...duh!
I get that for many people a dog park is their only alternative. But if you don't have to I wouldn't go back for your dogs safety.
That said: No you were not wrong to say something. I would have been jailed for punching the B (expletive).
And as they say " you can't fix stupid" and it sounds like that woman was stupid.
I'm sorry you and your poor dog had to endure the abuse of that mean, rude, stupid woman.