I haven't done it, but a quick google search says Coinbase is available in Mexico.
https://steemit.com/busy/@whatsup/real-use-case-for-crypto-help-me-brainstorm-sending-money-to-mexico
So that seems like it would be the easiest option.
Both get the Coinbase app and connect their bank accounts(assuming everyone involved has a bank account)
Then US end buys a fast to transact crypto like Litecoin or EOS and sends it, Mexico end sells it for fiat and has it transferred to bank account.