Based on Bithobo support's clarification earlier, it seems like he didn't inform them about the bug at all. He simply took advantage of it in order to win a lot of games and tried to withdraw his winnings which Bithobo refused.
If this is the actual truth, the ethical thing to do here is to ban him from their site and improve the security of the site drastically in order to prevent more such cases asap.
Let us see how will this pans out. In case, the site should just return the initial deposit of hauzenberg and ban his account afterwards. If they have potential bugs to resolve, they need to provide rewards for those who find bugs on their site so people will not take advantage if they found one. There will always be people like that.
I understand the "pay people to find bugs" type of thing, it is done by many people and many places and I agree that it doesn't sound that bad at all, it is basically just a way to make people work for free until they are actually useful, that way you do not pay some staff member to do nothing for not finding something, you only pay when there is actually proof of hard work. However I do not agree that anyone who finds one and abuses it should be given their money back, basically just either notify the website or be banned about it if you ask me.