The dad came over later last night and, apparently, he hadn't recognized me from his Ring and couldn't hear what I was saying so he just saw his dog freaking out and thought I was a stranger so he was yelling at me to get off his property because of that.
He said that up until they'd moved here a few months back, they'd lived on a farm and the dog had been trained to protect the property. The dog was getting old, the move confused him, and he was startling more and more easily and that led to him getting angry. He said the dog had never bit or even nipped before, but his daughter piped up that the dog had been nipping her a bunch lately. The dad thought that, when my son rang their doorbell to ask if the girl could play, it had scared the dog and he'd somehow gotten out of the house from the back and had come around all angry. He also said that they were probably going to have to put the dog down if it bites people when startled because it was startling so easy.
My son was much less scared/upset this morning, but now feels like it's his fault that his friend might lose her dog. I tried to tell him it wasn't his fault, but he's just starting to make friends after a year of pandemic isolation and I think he is a little worried (I am too) that he's going to lose one of his few new ones as a result of this.