The bookmaker must provide evidence and specify which exact rules the player violated. If the bookmaker loses money because its odds are value bets, that's its problem, not the player's.
IF the case is real, then yes, they must provide the evidence.
BUT, how do we know that the case is legit? It's easy to create forum accounts and say whatever.
We don't trust anyone at face value, that's the norm on this forum (AFAIK).
That's why it's really important that the individual who is being accused in a casino has proof of evidence to show, because there are many anonymous people here on the forum who make such complaints hiding their newbie account identity, and it's possible that the person behind it is a long-time player on this forum, there's a high chance that's the case.
And most of them are just criticizing other casinos, and whatever their reason is, that's what we don't know, that's why we just rely on the evidence they show, because without proof,
it will appear that they are just criticizing the casino.