From a purely legal perspective, you may not be required to pay the interest on the loan. The question for a court of law would be what the agreement was at the time that the agreement was made. If interest and finance charges were not contemplated, then you may not be obligated. We would need to know a whole lot more about the circumstances surrounding the loan for me to give you a meaningful guess.
From a friendship standpoint, it sure sounds like your friend did you a good turn in allowing you to purchase a car when you might not otherwise have been able to buy it. It sound like a real jerk move to now stick your friend with the additional costs of doing you a favor. There may be more to this story that I do not know that would explain why you are contemplating doing this, but that is my quick and dirty, first blush read of the question.
What did you agree to do? If you agreed to pay the interest & finance charge, then you are bound to this agreement. If it wasn't something that was discussed, a court would probably expect you to pay a reasonable amount of interest.
But really..next time you need help, what friend will help you out if you stiff the one friend that gave you money to buy a car? Friends like this are hard to find!
Hope this perspective helps!