I want to add to all the other good comments here that there should be no blame on the dog you were walking. That dog is in the right to defend itself from any other attacking dog, no matter the size. The description ”instantly latched onto the dog’s head” sounds like a reasonable action to stop the attacking dog. Of course, the leashed dog needs to know when to release the other dog, and that the hold on the little dog’s head should not be aggressive.

Prevention using intervention, spray, treats, etc. is always the preferred action. But then de-escalating both dogs is the next action. Assuming the little dog is still animated and the big dog is merely holding the head, not biting, the owner should be helping to quiet and control her pet. If she can’t do so, running in circles hysterically, she needs to remove herself from the situation. And then you or another confident person can talk both dogs down, eventually removing the little dog. The big dog should be praised for controlling the little dog without biting.

It would probably have been easy for the big dog to break the little dog’s neck, killing it. Because that didn't happen, the big dog was doing what it could to stop the attack, defending itself without counterattack. Seems legitimate to me. Of course, I don't have a full context here. Pitbulls are often unfairly blamed. They are as loving and good-natured as any breed. It is in the training and conditioning that any dog can become aggressive.