If they don't pay him out then they're effectively finished here.
They have a choice - pay him the 84 bitcoin & continue running a trusted site & make profits for an unlimited time
Or
Scam him & pretty much end their business with a destroyed reputation over 84 bitcoin.
For the above reason I'm confident they'll pay him out.
Maybe there's a third option: come up with a semi-convincing story about how OP cheated.
He played "live roulette", which generally means FJ paid some third-party company to spin a roulette wheel and broadcast video of it happening, while FJ took bets on the outcome. I don't know what assurance FJ have that the company spinning the wheel is broadcasting it live rather than with a 60 second delay. Such a delay would give them plenty of time to place bets on the number that already came up.
I guess this kind of cheating is what they are worried about, but I've no idea how you would go about proving or disproving it. I expect you just have to trust the third-party video feed provider that they won't play against you, felay the feed, use an unbalanced wheel, etc. Provable fairness cuts both ways.
In this scenario, the only way it would be feasible is to have the live casino provider issue a statement about how something was taken advantage I would think. There have been many big winners in live casinos posted here and it is perfectly feasible to legitimately win 84 btc. FJ must do the appropriate due diligence but FJ is trusted and if it is found to be legitimate, I do expect OP to be paid in full.