What should one do to purify the money of a person who used to deal with usurious banks, and now cannot distinguish between the origin of his money, and the loans and usurious interests?

Your friend must contact the bank to determine the amount of usurious interest and dispose of it by spending it on charitable causes. If he does not know the exact amount, he should donate what he believes to be its approximate value. As for the usurious loans he took and repaid, the money is his, and repentance, remorse, and a firm resolve not to return to such forbidden transactions are sufficient for him.