Loving all the self righteous people in here thinking the OP's dad did something wrong. From the OP's post, the bank put the 28k into his dad's account by mistake. He notified the bank and they didn't do anything about it. His legal obligation ends there.
The bank then waited 2 years to ask for it back and, in the meantime, the guy has put the money in a high interest 2 year bond to make money. He has not spent the money and, from what can be gleaned from the post, wouldn't have refused to give the original 28k back yet some people think he was in the wrong using it to make money himself whilst protecting the initial capital?
The guy was very shrewd and did the right thing. He did nothing underhand and certainly nothing illegal.