((((hugs Kaige)))) Poor boy! Since he's had so many good interactions with other dogs, he'll probably be okay. I know how you feel - about a week ago at the dogpark I walked in with Star & Sherman & within two seconds a big mastiff-mix was growling & snarling and had Star pinned to the ground! (don't know him, never saw him at the dogpark before ) When we walked in, he was just standing there calmly, his owner even had hold of his collar- I turned my back to hang the leashes on the fence, & wham! I was really shook up, too. But she was ok. He left immediately.
Your friend really should not have brought her dog without checking, but I'm sure she's figured that out by now. I hope Kaige gets back to his happy little self soon!