Even if it expired more than 6 months, it is a civil infraction. But if it happens again, no matter whose car you are driving, it is a 2nd degree misdemeanor. You should still tell the probation officer. Please click on the website, it does not cost anything. Thank you. "In your corner."
(a) Any person whose motor vehicle or mobile home registration has been expired for a period of 6 months or less commits a noncriminal traffic infraction, punishable as a nonmoving violation as provided in chapter 318.
(b) Any person whose motor vehicle or mobile home registration has been expired for more than 6 months shall upon a first offense be subject to the penalty provided in s. 318.14.
I believe it is 4 months or more which makes it a criminal traffic offense, and one which can violate your probation. In addition, you must report all contact with law enforcement, as the failure to do so can vioate you as well.
If the owner left the car to you he should have transferred the title to you. That is the law. And what kind of automobile insurance is currently on the automobile? Your license could be suspended for driving without automobile insurance. The tag info hopefully is less than six months which would be a traffic infraction. Have you looked at the expiration sticker on the tag? Just wanted to let you know that if you were in an accident and did not have insurance, under the Financial Responsibility laws you would be suspended until you paid off the judgment. This could potentially be $50,000 or more...much more. Drive someone elses car? I would never do it under these circumstances.
Good luck with probation, just explain what happened and call them asap.