This issue of your cousin regarding his bank accounts transaction money that as freeze by the government certainly have been found something like an unusual activity in that account. The Singapores Bank will not just freeze a certain transaction if they not find it suspicious. But well, it can only be resolve by your cousin. She/he is the one that can make things settle down. He just needs to comply what the banks requires to make process your cousins transaction money. Moreover, if there is no illegal doing or issues involve then this will end up all right and fine.
I agree on this, we can't blame the bank why they had to freeze your money maybe they think that very looks suspicious. We know the bank did not support cryptocurrency and they think that the money of your cousin was from money laundering or fraudulent. Anyway, if using cryptocurrency in your country is not prohibited then, there's no such thing to worry about. Just give them the right info. and tell the truth.