Send them a recorded delivery letter to the billing address.
I doubt they would have done anything with an email (could easily be spoofed) - to cancel an account you need to call & followup with a letter, but in this case hopefully the letter should suffice.
Did he try to call cancellations before leaving the country? If they don't have a record of being asked to cancel, he will be liable for services until 30 days after they recieve the call/letter.