The best thing you can do right now is pay for a qualified criminal
defense attorney in your area.
If you don't know any, then you can contact the Knoxville Bar Association's lawyer referral at http://www.knoxbar.org/lawyer_referral_service_pub.htm
A local lawyer will be familiar with the local prosecutors and can best advise you on what to do.
Simply writing a letter that is false isn't necessarily a crime if it is not under penalty of perjury. It could potentially be obstruction of justice
if it was said to police or the district attorney.
Simply letting someone without a license isn't necessarily illegal either. However, it could be if you were crimiinally negligent and someone got hurt.
Another issue is the potential to be responsible for any damage arising from the accident. Your insurance probably wouldn't cover it and you would be responsible under a theory of negligent entrustment (ie. you shouldn't have let someone without a license drive). That could be a big deal in financial responsibility.
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