If the lender has not pursued you for collection, don't do anything. If they do, refuse to take their calls or respond to their letters, and watch your credit report. If anything appears on it, contact the credit reporting agency and dispute the mark. Tell them the debt is not legitimate and that it was taken out by another person in your name. Do not offer any more details than that on the nature of the illegitimacy. You do not want to be making statements they can try to prove are false. You only want to deny the legitimacy of the debt and leave the lender with the burden of trying to prove it's legitimate (which they will likely fail to do).
Do not have any contact with the lender whatsoever. Any contact you make with them will be used by them as evidence that the debt is legitimate.