That's the way how they want to make you can't provide enough evidence to back up your words, but I believe 1xbit is a scammer, no matter what how someone convince me if they will change their behavior.
Anyway have you tried to wrote a complaint to their email?
support-en@1x-bit.com I think it's the last way to contact them since they're not quite active in this forum, even they're active they may not replied your complaint.
Even though 1xbit has the worst reputation, but anyone reporting a scam accusation must include evidence from photos or videos, at least his report is valid and of course there is no guarantee that team will recover his account and return the balance from his gambling account.
Sending a complaint via email won't get the response expected, "maybe" he will be blamed for not having evidence of his allegations, but an alternative solution is to file a report on the
Scam Accusations board thread.
He should pay attention to 1xbit reputation in this forum and know more about the influence of gambling reputation, so he should consider it before losing his deposited balance on 1xbit.