Answer:
ask the mechanic to document their findings
get a vehicle-history report online to look for evidence of odometer tampering
write a letter or email to the dealer, with copies of relevant documents
report the incident to the Better Business Bureau and your state attorney general
Step-by-step explanation:
First of all, tampering the odometer is a federal crime. The first thing you need to do is gather evidence and go to the police. You can do this by asking your mechanic to document any alteration and looking for other evidence online also helps. E.g. if you buy a car that is 10 years old and the odometer records only 10,000 miles, you should be suspicious (plain common sense).
I doubt that writing an email works, but you could be lucky and get a favorable response from the dealer. Or more importantly, it can be used as evidence that the dealer was aware of the fraud and didn't do anything to correct it.
Reporting this incident to the Better Business Bureau can help prevent that other people are tricked by the dealer, but as soon as you have evidence of what happened you should report the crime.