You have to know that just reading the tread before the story physically shook me up! First and foremost, I'm so relieved that Kaige is fine or I would have had a heart attack. Special, precious boy.
You have bundles of more experience than I, but it sounds to me, that you did the perfect thing but not coddling him (hard, huh?). And just like people (get back in the drivers seat) I think you're 100% correct to immediately socialize him with other dogs.
Having made awful socializing (or lack of) mistakes with Cody, and not quite sure of Logan's past, we try to take her everywhere. First and foremost, when we know another dog is coming over, regardless whether Logan's met them before, we always have her on lead until after the introductions and we all feel comfortable. I think that friend of your friend, owes you a huge apology for not asking your permission, assumming all would be fine and not following doggie ettiquet. I'm as dumb as a rock and even I know better.
The main thing is that our boy is fine. I hope, Sue, you're doing ok now too. See? If I had kidnapped him when I said I was gonna, this wouldn't have happened!
Endless hugs and kisses on everyone!