Maybe I missed a post of yours. If I'm reading this right...
Your ds goes to a friend and next door is a family with a threatening dog and an open gun case? hmmm, I'm not sure how you can stop them from going next door because that is a pretty logical move for boys playing outside. I probably wouldn't have a problem with that, but with the bity dog, geez. Our dog was like that and my Mom couldn't let him stay (fenced) in our yard if other kids were around. He was a german shepard and would lunge at any other child that seemed to threaten any of us. Kids playing looks like a threat to dogs. However, if someone were to say something to my Mom she would get defensive because she believed she had a good handle on the dog with other kids. My Dad's hunting rifles were in the house, too, but locked in a case with bullets locked in another area---still don't know where. We kids never went hunting, so never shot a gun and never touched them in any way. I think people get defensive about having guns in their house with children, too. So, how to address it? IDK. Anything you say could sound like an attack and if ds enjoys this child, you want to protect that relationship.
Can you go with? Is ds afraid of this dog and can you just say that in a casual way? Guess I'm not much help with ideas, but your fears are legitimate. Just don't know how much you can do about it and still have ds play with them.