Is it permissible to deal (buy and sell, lend and borrow) with a sinful person whose money is mixed with both lawful and unlawful earnings?

It is stipulated for lending to someone whose lawful wealth is mixed with unlawful wealth that it should not be known that he will use the loan for unlawful purposes. If it is known, then it is not permissible to lend to him, because it would be assisting him in what Allah has forbidden.