I disagree. The problems have all been related to resource guarding, and no one has yet to explain why the dog wouldn't suddenly decide to exhibit that behavior with a person. Giving his propensity to strike with little to no warning, with such intensity, I would be worried that if/when he decides he needs to guard from a person, that it will be really, really ugly.
I'm a professional trainer. The bulk of my business is related to dogs with problematic behavior. It is thought of among most veterinary behaviorists that inter dog aggression and dog to human aggression are usually two separate things. As I said, it is
not impossible for a dog to exhibit both...obviously. We see plenty of that in my profession and amongst the dog owning public. But to be overly concerned or to
assume a dog that exhibits inter dog (or other species) attacks, (prey, defensive, antagonistic, resource guarding and other types of aggression) will transfer this to humans is putting the cart before the horse or jumping to conclusions.
Dogs that resource guard against other dogs do not necessarily transfer that to humans. There are loads upon loads of dogs that have issues with other dogs, but accept humans around their stuff just fine and it never transfers, regardless if it's the same resource or trigger. I can provide hundreds of examples, even among my own dogs. Take one high valued item and watch one of them warn the other to stay away. I can walk up, ask the dog to give me the item and he's happy to oblige. With my recent dogs, I've pretty much conditioned them to give me things when asked, but not all my dogs in the past have I done that with and they too, were just fine.
This is not to say that a dog can not have resource guarding issues with other dogs
and humans. They do sometimes, like CP described. But typically, a dog that goes after other dogs doesn't
suddenly decide to guard against humans when all along, for years, he's been just fine with them. If it develops, I believe that it develops independently from the behavior exhibited toward other dogs...not that it developed
because the dog started out worrying about dogs taking his stuff. Dogs don't generalize well and many types of aggression toward humans and aggression toward dogs are thought to be viewed separately by dogs rather than melded into one single, condition/response thing. Territorial aggression may be a more generalized thing to some dogs. If a dog constantly has things taken from him by humans without some kind of reward or trade, sure...he may well develop resource guarding. We see that a lot. But just because a dog guards his stuff from other dogs, does not warrant fear that he'll necessarily or probably "decide" all of a sudden to guard against humans if he's never shown trepidation to humans before. They're not as complex cognitively as we are. They don't think..."well, dogs have threatened my stuff, but no human ever has, but they might, so I better start attacking humans too, just in case." They probably think, "Dogs have threatened my stuff so I better beware. Humans haven't threatened my stuff, so no worries." Period. End of story. If humans have threatened their stuff, they may indeed become defensive.
Caution and prevention is always, always a good idea...conditioning, exercises trading something better with a dog for something he has possession of helps raise the odds that he'll not develop a threatened feeling. Also, people don't read dogs warning signs well enough or early enough. That's understandable. And a lot of dogs don't read "dog" very well and they wind up pushing the dog beyond his comfort level well before an attack happens. We just don't see it coming and it looks like it's "out of the blue" when all along there have been signs.
One of my dogs, Jose` will guard the Dremel (yes, you heard me right. lol) against the other dogs if they merely walk past it if I leave it out. They couldn't care less about it, but he warns them just in case. LOL. There's no attack...just a little growl to tell them. If I or any other human picks up the Dremel, he couldn't care less. I can think of lots of examples where any one of my dogs may guard a mutually coveted item, such as a deer leg bone one of them found in the woods...more than once. If the other dogs are near, the one in possession will guard it. "grrrrrr." And the other dog(s) will veer away and not push the issue. If I go up and want to examine it for anything gross like worms, before giving it back to the dog, there is no sign of concern...never have had that problem. Now, if I had taken things off my dogs constantly or punished them, then taken the item, that could have lead to a different story. But....just because a dog guards against other dogs taking his stuff does not necessarily cross over to resource guarding against humans.
I disagree with the behaviorist who utilized an aversive response, such as a shock collar in conjunction with anything even remotely related to aggression. That is thought by most modern behaviorists to be a serious no no because dogs learn strongly by association. There's too much risk of fall out...if not immediate, often later on. That she waited for the dog to calm, then reward is fine and dandy. But no one knows for sure how much time must elapse for individual dogs to lose or miss the association. Studies show that shock collars raise stress hormones and in a dog that is already stressed, I disagree with piling on more stuff for him to worry about. The stress hormones, adrenalin, the higher respiratory and heart rate does not drop off instantly or in the time elapsed between the shock and the reward. It is good that everything came out well for your dog. It's just that I feel the risk of back-fire is significant enough to avoid aversives altogether when dealing with aggression issues of any kind. There are too many other ways to handle behavior problems without punishment. JMO.
Anyhow, with this particular, theoretical dog, he has not shown resource guarding toward his family. I would recommend nevertheless to implement small practice sessions where the dog gives a mediocre valued item in trade for something better and then give back the item and repeat as a fun game. I would recommend they practice with a variety of things, graduating from lower value to higher and the traded item respectively proportionate to the item traded.
Do I think that
because he's guarding against other dogs, he's going to start guarding against his humans? No. I think that he could develop a human resource guarding issue if he has not learned to defer to humans for his resources. It's always a good idea to keep practicing that.
Even though he's fine with children, they can take part (supervised and if safe) in having the dog defer to them for food, affection, toys, getting to go for a walk or go outside etc, etc. It would just strengthen the relationships in the right direction. It's always better to put the odds in one's favor by covering all bases because we can't predict how any individual will act, depending on stress levels, changes in environment, a rotten day, novel stimuli and on and on. Many conditions or variables can affect the way we all behave. If they ever feel the dog has nervousness regarding children, then that scares me too much.
If these people could prevent him from biting people, build a fence, keep people out, supervise this dog better, use a muzzle when out and about, it is possible he could have a good life and people could be kept safe. But after repeated accidents, I have my doubts about their ability, which is strange because it sounds like they've put in the time for obedience training and so forth. If they could socialize him more, starting with known friends, associating high value treats with them, working up to less known visitors, he could improve a lot. I definitely recommend that they don't tie him up, but build a fence instead. Lots of exercise increases happy hormones, which should also help. And of course, (I forget what's been said) but a vet's visit for anything off medically.