Doesn`t sound like it would be that had to get the guy owning money to sign a new doc. He is still in communication, still promising to pay. Bigfit, tell the guy you understand, lets start new now, no back interest, drop the interest rate down to 5% because in this environment that`s fare and good return, just get him to sign a new contract. Also if the money was not used for what is was supposed to be used for you bring on fraud, wire fraud, a whole bunch of stuff, not just "bad debt". I doubt the money went where is was supposed to. Otherwise it should have yielded/returned SOMETHING.
Before suing the guy you gotta know what his assets are. Judgement is useless if he`s got nothing to take. Also, is it possible he did this to others, that changes things a lot. What were you loaning the money for, what was the investment premise?