My friend from colombia is coming to usa for 3 week visit on her tourist visa, already obtained. It is my understanding that she could be denied entry into usa if she does not have appropriate amount of cash on hand when asked by immigration officials.
Is this true ?
Who determines how much money is needed ?
Will a credit card be accepted ?

Credit cards are OK, normally immigration officers ask for return plane ticket, only when they find someone suspicious then they ask for other documents or search the luggage.