It could happen, there are owners of Caddys with almost 700,00 on the clock that are still going strong on the original bits. The transmissions have been rebuilt however...
I know there are 3rd gens with 400,000 on the the clock over at 3rdgen.org
I have no idea who has the highest mileage though.
Does she have some sort of proof of the mileage though? It would be a lot of oil change receipts for a start...