That grey dude is going to get busted betting sums like 300+ bitcoin on your site.
He's really pushing his luck, one day he's going to lose it all mate, that will be a good day for you

Anybody who bets like that will meet their fall eventually.
Maybe if he continues then yes. But since he is betting 300 BTC at 97% , he has almost got a lucky session so far being close to 300 BTC profit. From what I have seen, All it takes for the big bettors is one loss, and they lose a lot more just trying to cover their losses.
The luck is still on his side, even know he got more adventurous betting ~179 at 88%. I wish him all the best.