absolutely the bank could and willa sk for the money back, unless the depositor intended for her to be the recipient. Even if it takes them a year to sort it out, they would still have the legal and moral right to demand the money back. Even more so because she realises it is a mistake. I would bung it in a high interest account, and give it back when they ask for it. It could just be someone has put the wrong account number in. They will be able to trace the moneywhen the person realises there is not 1000 pounds in their bank account that there should be!