You need to talk to theoffice/adoption manager there and report the situation. This woman is overstepping her boundries by making a judgement like that. To back up your father-in-law's condition you may want to get a doctor's note as well.
What is the name of the other rescue/person? It may be someone we know. Keep in mind that rescues are usually run by volunteers who have full time jobs and families as well. They don't always have time to respond to calls and email right away.
Have you checked
Petfinder.com: Adopt a pet and help an animal shelter rescue a puppy or kitten. ?