Tell the person you heard the "inside information" from that he would have to be the one to blow the whistle. He'd probably be protected under the "whistle-blower" laws, so there's nothing for him to worry about. Unless the lender sends Rocky and Vito over to snap his legs off at the knees.
Also, you don't define what a dirty loan is. Loans made on property they know not to exist would be dirty. It might look dirty, but sometimes there is a legal and ethical way to lend money and get one helluva return on your money from someone it looks risky lending moey to. That's not dirty, just underhanded and slimy. Do your research and find out which is which.