Normally when you make a deposit to a gambling site or exchange . They give you a dedicated deposit address. You send funds there. When there is a deposit detected the casino credits your account. Depending on how they handle cold storage it might sit at the deposit address until someone else makes a withdraw.
So problem is, he made a withdraw, and later sent BTC to that withdraw address. That withdraw address might of been a deposit address for somebody else. That person will login into their account, see some free money and just assume it was some gift and use it.
Either way, he should contact the support and they might be able to send it back to the source address if it wasn't already spent.
Exactly, that's the point. I asked for support and also sent a PM to TheQuin. I hope the wallet to where I sent my BTC belongs to Freebitco.in. I think it belongs to them because (i) they sent me money through it and (2) the balance yesterday was around 120BTC
Anyway, thanks everybody for your quick support