Hey he asked for the "worst" possibility. And it really isn't that hilarious, and in fact it could be rather serious. Statutes in some states enlarge the scope of larceny to include embezzlement and false pretenses. In North Dakota, for example, the crime of larceny includes: "stealing, purloining, embezzlement, obtaining money or property by false pretenses, extortion, blackmail, fraudulent conversion, receiving stolen property, misappropriation of funds, swindling, and the like".
If that were the case where he lives, it wouldn't matter if he took the money purposely or not. The fact remains that received money and subsequently spent said money when he knew it was not his (or at the very least questioned it). That action certainly wouldn't help his case, either.
If it were a bank error and the money wasn't spent, then that would be easy. But, since he spent a good portion of it, he could be in for some serious retribution. This could include not only paying the money back, but fines and possibly further punishment.
These scenarios may sound extreme, but they are not out of the realm of possibility.