Bank has no way of knowing he was gambling with crypto, all they can see is funds being transmitted to crypto exchange and back, depending how he transfers the money.
In these instances, banks have to be very risk adverse. The potential for fines and sanctions is huge, they have automated processes to flag anything that looks suspicious and it has to go through manual review. 99% are reviewed and approved, although it’s the 1% that are truly proceeds of crime/money laundering. If it’s payments and receipts to and from crypto exchanges, then that’s a lot more likely to prompt a review.
I’m not suggesting by any means that this is the way things should be dealt with, however I’m only iterating what does happen and how banks and financial institutions perform their transaction monitoring. Not my opinions, just the processes they follow.