A provably fair crypto betting game would have it's code on Github i.e. open-sourced. That way a developer can easily confirm the authenticity of the claims. Another way would be to use a provably fair tool to check. These tools can be found online.
A friend lost a chunk of his crypto investments to a scammer that claimed his game was provably fair. The owner of the website waited till the website was popular, to accrue more victims. Then he simply closed the website and vanished with thousands of dollars belong to people.
Don't play because others are winning!
That's another tactic the scammer used. He usually gave winnings to a circle of accounts that were managed by him. When you try to compare the losses of the accounts to wins, the wins would be more. He did this to entice his victims.
Be safe!
You did not name the name of the betting site, although it is now closed at least we can have a reference on what kind of betting game that is not provably fair, you are right people should research if the site he is playing provably fair or not and if the site can be trusted to pay winners of their games.