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.
Any transaction done between two persons in respect to bitcoin to me has no proof of any sort, not even a ledger or digital receipt. Even though there is any sort, the bank will not see it as proof. Advise, kindly review the above forum members suggestions and find the most suitable one for a guild. Exchange base transactions are preferable to avoid such embarrassment from the Banks next time.