I think this is common at bc.game because they often ban accounts when players get big wins. People who actively play in bc must know this. One of the best ways is open case in casinoguru and trustpilots, on this forum also
I don't think a big win is a reason to close a user account, the problem is when the users abuse the casino to win big, i have seen some users creating multiple accounts to abuse the bonus and after the win is when they get banned and start complaining. If a user break the rules the casino is in their right to block the account, and users must understand this rule, when a user deposit on the casino the money now belongs to the casino and not to the user.
Agree since somehow we could see that they are paying on past huge winning made by their players before. So maybe there's some issues happened that's why he encounter this kind of problem. That's why we should not try to break their rules and think about that the casino won't notice it.
He better communicate with their support and ask for latest development of his case since provably that he would get an answer towards what really happened. But if he think the situation is worse in his side and he really didn't do anything then maybe posting a proper scam accusation might help his case.