One thing you need to know is that if someone gives details of his account to someone else on his own will- then Oasis prolly won't do a thing to help him. It's his own fault like you said.

https://forum-narutoen.oasgames.com/page/show-post-32694-1.html

Beside that- it should be reported to Customer Service. https://support-center.oasgames.com/?lang=en-us