Invoice has been hacked

I have just been notified by a client that they have received an overdue notice with invoice attached to it from the MYOB system (which would be correct) - however he called as our bank details had been changed. It seems our invoice was intercepted. He did say that he had been notified recently that someone in Washington had tried to get into his bank details, so I do think it is his end that may be the problem. He has had invoices from other companies emailed around the same time as ours and their details are all correct. I then emailed the invoice in question to him again and it now is also correct. Not sure how this has happened, I have changed passwords to everything but was told I should also inform MYOB, as I don't have hours to spend waiting on the phone I was hoping this would get to them... has anyone else had this happen recently and did they have any ongoing issues or repercussions from this?