Many state's laws provide that you cannot lend money at an interest rate in excess of a certain statutory maximum. This is a "usury limit." The usury limit which is stated as the general usury limit is the rate that can be charged by one person or corporation to another, in other words, if you lend your next door neighbor $ 100.00, the rate stated is the limit. To charge more you must get a banking, pawnbroking, or whatever license. Banks and credit card companies, as you know, can charge a higher rate of interest.  In California the limit is as follows:the legal rate of interest is 10% for consumers; the general usury limit for non-consumers is more than 5% greater than the Federal Reserve Bank of San Francisco's rate.Please get your friend's Dad some legal help ASAP.  There may be a statute of limitations question but if the payments are on going it may be extended or "tolled."  Good luck.