In that case he wont really have any option but to contact support and do ask for further instruction
regarding on the transaction.
There is only one option left for OP if the aforementioned option isn't viable, which is signing a statement from the address the coins were sent from. If that's not possible either, then I hope for OP the amount is very small.
I really hope it's not sent from an exchange, because that means it's officially impossible to regain access to the funds. That's why you should always first withdraw to a freshly created address of yourself, and then mix it.
I know beforehand that an exchange won't sign messages to help someone who really needs it, and even if it is possible, an exchange might not be interested to cooperate. It's time consuming and doesn't make them win anything.