thank you everyone for your responses, all advice's taken and will guide him accordingly.

if my cousin did bitcoin transaction without exchange to that person from Thailand then is there any sort of proof for that bitcoin transaction done with that person ? any sort of ledger or digital receipt perhaps that he can produce from that transaction so banks feels it genuine bitcoin transaction and not illegal money or money laundering.

my other concern is why do banks need to know how my cousin got those bitcoin and produce source of income for getting that bitcoin.

my cousin sold his bitcoin to a guy in Thailand and that person made direct payment to my cousins bank account in Singapore from his Thailand bank but now the bank in Singapore is asking my cousin to produce his income source and has freeze that transaction money in his account. my concern is does the bank have that authority to stop my cousin from taking money out from his account and give such reason or excuses ?
is there any legal solution ? is it possible to sue the bank ?