The only way it would cost zero is if he got paid in bitcoin and his family were able to spend the bitcoin in mexico instead of reverting back into Peso. That ain't gonna happen never, cause the US and Mexico would never tolerate their currencies being replaced as the default currency for the country.
Maybe not the government, but look at this article:
Want your paycheck in Bitcoin? Get paid with the BitPay Payroll APII know this will most likely not happen with Mexican immigrants for many reasons, but this is just an example that you don't have to depend on the government accepting it to use it. You just need that more services emerge that make it easier to use Bitcoin on a daily basis.