Infiniti G35 Sedan 2005?

Hi, I have a friend of a friend who is selling his g35 sedan for $8500. It currently has 120,000, but he has a certificate stating it has a new motor with 40,000 miles on it but I saw from the dash board it still says 120,000 +. Is this a good buy? Should they have reset the mileage clock? Anything else I should know?