Owner won't believe me when I told her dog was aggressive? [closed]

Hi Natalie, Sorry to hear you are having such a hard time with this puppy. I understand your frustration when the owner thinks that their dogs are absolutely perfect and would never act up. lol
A few things that I would do differently....

You mentioned that the puppy knocked over a man. You need to make sure that you have full control over the dog at all times. Do not let the dog get close to people or other dogs. It is your job to keep the dog safe, and when walking him, the people around you safe from your dog. If you see someone approaching you need to cross the street, or walk the dog early in the morning or later at night when there are less people out.

You mentioned that the owner has never seen this behavior, make sure you are not putting negative tension on the leash and that you are not contributing to "leash aggression". Some dogs can feel your nervousness when walking and when you tighten down on the leash it will in turn actually cause leash aggression.

I would switch from using his collar to using a "gentle leader" when walking. I find that with a "gentle leader" you have more control over the dogs. There are tons of different leaders that you can try, you just have to find which one works the best for you and the dogs. (I personally have several different leads on hand. This saves me from the dogs pulling me down the street and insures that I have full control over the dog when walking.)

Best of Luck!

Probably the dog acts differently when walked by the client than with you or they rarely walk him? Even if the client is in denial what you have observed isn't proper behavior and a trainer would possibly help. All you can do is be honest in your observation with the owner and let them decide their course of action. And you probably shouldn't accept this dog again in the future. Best of luck

Either the owner is in disbelief or actively trying to avoid liability if their dog hurts someone. Is it possible to avoid walking the dog while you're boarding it? (Can you play fetch on a long line or in a fenced yard?) I can imagine a number of scenarios that would be inconvenient or dangerous for you, like if you encounter a stray or off-leash dog or come upon an on-leash dog without expecting it (or a little dog on a flexi-leash for example). In addition, dogs with leash reactivity can turn on their handler out of frustration. I would avoid walking the dog if you're not familiar with dog aggression issues.
Also, if the owner is absolutely refusing to believe you, I wouldn't push it. You've already informed them of the issue. You can't control whether they believe you. If they express their disbelief again, you can simply say "I'm so glad they don't do this with you!" or "He must feel safer with his mum then!"