Has your friend got the cashout already? And now the only complaint is about locked account with no balance? Or the cashout is still pending?
OP's friend was able to withdraw 3.5 SOL, but according to the OP, his friend still had $1100 in the account before it was blocked, so from all indications, the friend is owed $1100. If the balanace was empty, i don't think they will bother, they will just move on to another casino and play there.
most people bet in round numbers like a $10, $50, $200 bets etc...
but if you are actually doing arbitrage with a calculator to maximize your profit, your bets would look like something like $22.5, $134.7, etc... which is a dead giveaway.
Oh, i get it now. Meaning it is quite "easy" for a casino to spot arbitrage betting.