Does the dog react to voice or touch?
When someone intervenes, how do they do it?
Your friend should be the one by the way doing the intervention. It is the handlers responsibility and not others. By others doing it, it does not teach him who is in control which should always be the main handler.
He sounds like a dog parks worst nightmare. When I had dogs that could go to the DP (either my rott or my GSD), if I ever saw an unruly dog, I usually left with my dogs because you can never 100 percent say you always have control. It is 'normal' pack mentality for dogs to collectively attack an already attacked dog. It's ugly and it can be very disasterous.