Does your friend speak Spanish? Is anyone helping him?

I recommend that your friend obtain the services of a lawyer. I highly recommend the services of Lic. Lucas Nava Brito here in Zihuatanejo for a matter like this. His contact info can be found on my Professional Services page.

However, YOU cannot drive your friend's imported car! That is definitely illegal

From Mexican Customs:
Importación temporal de vehículos

¿Qué personas pueden conducir el vehículo?

Los vehículos podrán ser conducidos en territorio nacional por el importador, su cónyuge, sus ascendientes, descendientes o hermanos siempre y cuando sean residentes en el extranjero, o por un extranjero con la misma calidad migratoria. Cuando sea conducido por alguna persona distinta de las autorizadas, invariablemente deberá viajar a bordo el importador del vehículo.

That says a foreigner with the same migratory status (resident outside of Mexico) is authorized to drive the vehicle, and implies they are authorized to do this without the importer of the vehicle on board. That is implied because the second sentence says those not authorized to drive it must travel with the importer on board, which implies that those who are authorized (listed in the first sentence - Mexican citizen family resident abroad, and foreigners) do not need to have them aboard.

In other words, if he is not a Mexican citizen, then this sounds like corruption plain & simple. However, he said he is "Canadian/Mexican", and usually if you are a citizen of a country then you get treated like a citizen while there, and shouldn't be returning as a foreigner. But if he just means he is ethnically Mexican, then he should be ok to drive it, based on the text above, which is nearly identical to what I remember actually being printed on the back of the car permit hologram document they give you.