None of this sounds like this was a real police officer.
Mr. Driessen is a former Deputy DA in Orange County with over 8 years of criminal law experience. Nothing stated on this site shall in anyway be construed as legal advice, or as creating any attorney client relationship. If you would like to hire Mr. Driessen, feel free to contact him at www.theocduiguy.com.
Your dad's placard has a number assigned to it. He should be carrying some paperwork from the DMV identifying him as the owner of the placard. His number will be on there. The DMV will replace the placard for him. Tell the DMV to invalidate the number on the stolen placard - well, maybe they'll do that without being told how to do their job.
You might also want to make a police report. I'm guessing the police will tell you to file your [dad's] report online. Will your placard be returned? Probably not. But it will alert the police that this clown is skulking around that area conning disabled people out of their placards. Who knows what else he was up to?
If you really want to stir things up, the SF Mayor's Office has a Disability Rights kind of advocate. They intercede with police sometimes. [I did this about a year ago and it was soul-satisfying to get a letter from the station chief promising that his cops wouldn't park illegally anymore.] The Mayor's Office might be able to pressure the PD into adding more police in that area or being on the lookout for this kind of activity.
If you were issued a citation for the misuse of the placard, you will want to go to trial and plead not guilty in front of the judge. Bring your father and his caregiver to support your testimony.
You ask if the tickets are "valid?" If they were issued by a law enforcement agency, yes they are "valid" but you may not actually be guilty of their alleged charges. While it would behoove you to retain private counsel because you will have a couple of court appearances, you can try and represent yourself.
This is NOT legal advice. No attorney-client relationship exists.