There is nothing from Bittrex because Bittrex probably was not hacked... hacked were email accounts and attackers probably ask for forgot password from bittrex or so. But these mails were probably imediadly deleted because he havent any mails from bittrex in mailbox.
But attackers must knew these mails are connected with bittrex, maybe leaked all emails from bittrex?!?!?
I also think that the victim could be the hacked one, maybe his computer is compromised. Otherwise all other accounts would probably have been hacked too.
If you enable 2fa function then even though hacker gets your login access they can't do anything in your account because they need access to your mobile phone as well. That's why before adding funds to any online accounts it is better to enable your 2fa function to safeguard your money.