They could use hawala, a value exchange system which has been around for centuries. Fees are reasonable, the system is extremely trustworthy and use is not limited to Muslims.
The TransferWise model is the future of global banking imho (disclosure: I work in the financial infra space). One user account, the ability to hold a variety of currencies simultaneously, and banking info for each currency to plug into financial rails for said currency (routing/acct in US, IBAN in the EU, etc).
Turns out money is just rows in a database and ISO 20022 messages, queued and processed.
I had enormous problems trying to do anything with Transferwise from Cambodia. It's still plagued with problems left over from the conventional banking system.
This loops back to one of the few things cryptocurrencies do well: they're extrajudicial, so they're often used for illegal transactions (drugs, ransom, bypassing sanctions). Bitcoin just happens to be really bad for this since the blockchain is forever.
