"" He may be grieving and is angry because he thinks you took his friend away.""
Let's examine your theory of who is a "know it all."
The op has a young maturing dog who is showing aggression and biting. Other than you, not one person declared that, sight unseen, they knew exactly what the issue is (blaming owner and mad.) This is an escalating dangerous problem. So I will say it again.
The only "know it all" is the person using their omnipotent power to declare as a FACT that their dog blamed them and that is the problem. Your response is reckless. It endangers the dog in the off chance something medical is going on, and wastes the op's time....precious time should this dog bite someone who registers a complaint. No rescue will touch him and the owners can be sued.
If you can't handle someone mildly disagreeing before, this one may send you over the edge. Let me point out, since I am sure you're missing it-- I'm still not telling the op I know what her dog needs.