That particular person can always press charges and make a police report if they like, however the grounds in which any formal charges will be filed against you carries a much higher burden. To issue formal charges against you, the local prosecutor would have to have probable cause to believe that that person's library card was indeed stolen, and stolen by you. If so, the prosecutor could charge you with Larceny. I would not worry too much as police and prosecutors do not interject in small items like library cards too often. To be on the safe side, I would contact a local criminal defense attorney in your area to see if there have been any warrants issued against you and to discuss you various legal options in this matter.