Last time I contacted with Stake support in other to recover access to an account they were pretty straight forward and gave the option to sign a cryptographic message with one of the addresses I used to deposit money into the account, as a proof of ownership, I don't know whether that continues to be an option, if so, then perhaps he could try to contact support through email and provide such proof of ownership to the administration of the casino.
If he got hacked or fished, then he should elaborate on the method scammers used to get him, so people in this thread (who specifically have stake accounts) can be extra careful not to fall for those scams.
I also helped my friend take over a hacked account and the support team provided some questions for example, wallet address used for deposit/withdrawal, IP Address, Device, Regis date and some other questions well, from all the answers, the support team can determine whether we are the real owners or at least have owned the account. If you cannot answer all the questions due to loss of access, the last method is request re-KYC on the account for example live KYC through a third party or hold a paper with a date. In another scenario some people have sold their accounts and then tried to take over and usually cases like this are for accounts with high VIP level Platinum up.
