I can't imagine it's uncommon at all. It runs from people just buying harnesses/vests for their animal that says "Service Animal", to people who have a friend who is a doctor (not even a psychologist or related field) who will write them a doctor's note proclaiming their animal a service animal. I know both of these types of people, and I know they've brought their dogs on planes based on this.
It's disgusting and makes me uncomfortable whenever I'm at an establishment with them that usually doesn't allow pets.
I would imagine in the latter case that the doctor would not be pleased to hear that their friend told you this story, as something like that could be grounds for disciplinary action on their license. Totally unacceptable to sign false documentation and seriously calls into question their professional integrity.
It's disgusting and makes me uncomfortable whenever I'm at an establishment with them that usually doesn't allow pets.