Hey Brandon!
I went through the same thing with Tulsa.
I adopted her at 1 year old and was warned about her leash aggression, her dislike for men and children, and her dislike for other dogs.
95% of it was the leash.
I took her to the dog park everyday, she could run off leash and approach people and other dogs on her terms, but I was never so far from her that I couldn't stop an altercation. Yes, she did have some, she wanted to establish herself as the 'Queen' of the dog park, and would bully other dogs to submit to her. She wouldn't fight or bite, she just used her size and attitude to establish herself. Sometimes it worked and sometimes it didn't, sometimes the dog she was trying to dominate wasn't buying into the program and would jump on her and show their displeasure. It only lasted a split second, and there were no wounds to either dog. Tulsa would fall back, regroup, and show the proper respect to that dog. More often than not, they would be playing together before we, or they, left the park.
I'm thinking that Bava allows Mowgli to be the boss because it's the path of least resistance. (Newfs are so cool in this respect.) I got my food, I got my water, I got my water, I got my water...oh look! A toy I can drop in my water!
Obviously I am a proponent of 'Off Leash' dog parks, the dynamics of a dogs socialization and desensitivatio n are of extreme importance to dog owners.
When my 90+ lb. GSD mix can play with a 4 lb. Mojo, and let it chew on her neck, well, my job is almost done.
And Ang,
If your friends GSD doesn't learn some manners, it's not your fault. And I agree with your statement that dogs are not naturally aggresive. It's lack of socialization at an early age that causes fear in an older dog, and that fear can lead to uncontrolled aggression, or unexpected aggression. Tell your friend to be on his toes.
John