The issue of fines is specific to the laws/code of your municipality. You may wish to consult with a local attorney on that issue. In general, however, I believe your best option is to contact the local police department and indicate that you have an unregistered car on the property that needs to be removed. Depending on the department's procedures, they may require you to sign an affidavit that indicates certain items of your personal knowledge cocnerning the vehicle. Usually, there is a waiting period that the police will invoke where they give the owner notice (via a sticker on the vehicle) and an opportunity to remove the vehicle from the property. If not removed, the police typically tow it to the impound yard at the owner's cost. If abandoned, it could evenually be sold at auction.
You indicated that the owner lives with you. Is this by lease or verbal agreement? Have you given the owner permission to place or store the unregistered vehicle on your property. If you lease to the owner, do the terms of your lease address vehicle parking and a requirement that all vehicle must be registered and possess a current & valid plate? You may wish to also address this issue with a local attorney to protect yourself from possible liability under any renting agreement.
Good luck to you in resolving this situation.
Disclaimer: The above response is not intended to create, nor does it create, either an attorney-client relationship or an ongoing duty to respond to questions. It is intended to be solely the educated opinion of the author and should not be relied upon as legal advice. The response given is based upon the limited facts provided by the inquiring person and additional or differing facts might change the response. Attorney is licensed to practice law only in the states of Illinois and Michigan. Responses are answers to general legal questions and the inquiring party should consult a local attorney for specific answers and advice.