Always activate the 2fa security wherever the option for it is available.
Such instances will keep happening and we have to set in every security measure available from the site.
Its still not guaranteed but that is a scenario very likely in crypto-currencies.

Also dont use the same password on multiple sites.

I am very sorry none of my suggestions are actually of any help in the current scenario and its unlikely anyone can do anything if the transaction has already taken place and balance has left the site.
But you can use this experience to not fall for similar problems in the future.