Informed by DMV of odometer discrepancy.

A friend of mine recently purchased a used car from a dealer. A couple
months after purchasing the car, they were informed by the DMV that a
discrepancy exists between the odometer reading on file at the DMV and
the milage that was reported by the dealership (with the first milage
number somehow being *larger* than the later odometer reading).
Obviously, this could present a problem when the time comes for my
friend to sell the car. The dealer isn't being cooperative in getting
the situation resolved. Does anyone know what options exist (through
legal channels or otherwise) when you find yourself in possession of a
car that may have had its odometer tampered with, and the entity that
sold it to you says "Sorry. Can't help you" ???

Many thanks.