My foster daughter is driving me bonkers

I would find the service dog your FD needs, then trade it out for this yippee one. Sounds like she will attach to a new one just as well. It is too weird that the one that you found suddenly needed to be introduced to another dog to see how it does...that doesn't even make sense. Dogs are trained for months before being qualified as service or companion dogs. It sounds like the owner didn't want to give the dog up and staged it so that this dog reacted...just saying. I've known a lot of people who have trained service dogs and they take them from a pup and raise them around other dogs, children, noise, everything that they will need to protect, lead, guide their forever owner.