I contacted myetherwallet but have received no response.
Any ideas?
Yeah, the probable fact is that there was phishing, sounds like you sign up for every thing that comes in front of you and that means that you surely have given your accounts to more than a few people. That is something that you will see in the account details, so that is also a place to look and I am sure you will see where it has gone. Or, you just need to give it more time to get confirmed, or maybe there were security issues that you did not comply with on the one end.
That would be the saddest thing happened to him if he had fall on the phising mew site but he shouldn't conclude some things like that first since he need to track back on what he do and check on the block if his transaction has been confirm on coinbase nor in myetherwallet, But if there's no correct records shows that he sent on the correct address then he must be worried since there something fishy happens on his eth balance sent.