I have sympathy for everyone involved.
Of course it was horrible for Kate and Reuben. But, as Rachael says, if her fear were so bad that she couldn't control her actions, that is something she deserves sympathy for. If it were one of our dogs who was lunging and biting from extreme fear, we would understand. As that dog's owner, we may shout at the thing causing our beloved dog such terror, in order to get them to move away. Humans are animals, too, and can react in very much the same way.
It may also be that she doesn't have such an extreme reaction every time she sees a dog and that a specific thing triggered her this time. I am afraid of flying. Sometimes I maintain my composure, other times I turn into a shaking, blubbering wreck. I don't know what my trigger is because there seems to me to be no rhyme nor reason.
It may also be that she has some underlying condition that caused this reaction: mental health issues or physical trauma. Without having all the details, we probably shouldn't jump to name-calling, even though it's natural that we want to, to protect and support *our* loved ones, Kate and Reuben. Even if she were nothing more than stupid for putting herself in that position, it sounds as if it was a horrible experience for her, too.
I'm certainly not saying I follow my own advice all (or even much) of the time, but taking a step back and considering that there may be a bigger picture can sometimes help to disperse the stress and anger towards the other people. There's already too much of that in the world.