Video video video!
Most people walk their dog at the same time every day. So have her record the off-leash walks, about a week/two weeks worth. Then go in with your complaint to the BY-Law office. And request to be followed up with after the investigation.
Something will be done. You just have to show that you are serious about the complaint and have evidence.
Our By-Law complaints are all online and they make availability for pics and video for this reason.
EDIT: No matter how well trained your dog is, you can not control another dog. ALWAYS leash your dog for THIER safety. If a dog, off-leash, come to attack you and your dog, with your dog also off-leash, you are not catching either dog while they fight with each other. Ans most dogs who attack while you are walking your dog are looking to get your dog, not you. I have had to do this many times, but putting myself between my dogs, who are held by the leash tightly behind me, and the attacking dog. The attacking dog wants nothing to do with me. We play the dance game of the dog trying to get behind me, and by that time, the owner is usually there to restrain their dog. Only once did I have to hold my dogs behind me, and basically straddle the attacking dog with all my weight. That one was scary but no dogs injured. I use leashes and harnesses for this exact reason. To control my dogs when another dog is out of control. All my dogs are trained but it is not about training. It is about safety and you can't do that without control.