I would suggest for your FS member to send a support ticket in. They can try and request an investigation to see if their account was indeed hacked. I suspect the results of this will rely heavily on that member proving they didn't share any of their login and password information with anyone.
As secondary note, since this has happened with several players already in this game, did that member login, went to complete a quest and did not receive the rewards? hence thinking a hack had occurred. Further, which has also happened, not hack related, people have logged in to find inventory gone, yet another bug/ glitch. These type of scenarios will require a support ticket as well.