If you feel threatened you do what you have to do to protect yourself while trying to inflict the least amount of damage on your attacker (in an ideal case anyhow). To me it seems like that is what she did if the dog acted in a threatening way. She did not shoot the dog, she kicked it.
By the way, like L-Pink pointed out, the "average person" should be able to make the call whether the dog is attacking or not. If it's hard to tell or there is no time to size up the situation, then I'd fault the owner of the dog, since the dog wasn't on a leash.
(ps. I'm not talking about who's
legally at fault, just the common sense view of a situation like this)