It is your decision to make. To the best of my knowledge, you get no reward for turning in someone to the office of the United States Trustee for bankruptcy fraud, but the IRS often does pay rewards to whistle blowers. Hope this perspective helps!
Well..the question I would ask is whether you benefited from his fraud. I cannot tell from this question. If you did, It may be better to come clean since you just discovered it. If you have no involvement with it, it is all up to you. In any event, you should talk to bankruptcy/tax specialist. If he /she determines that you may have criminal exposure as a "partner", he/she may decide that you will need to be careful as to how you decide to approach the bankruptcy trustee or IRS. I believe that caution is in order. It is very important that you talk to the correct specialist.