collection agencies? restrictions? allowed?

Are collection agencies allowed to access bank report records? What type of limits do they have?

We have a customer that decided not to pay, and then CB they're account with us on american express. Can the collection agency pull up they're assets and records? If the collection agency went to pursue a lawsuit against them? To show that they did have money in they're bank account...and withdrew it knowingly that they were going to disappear on us and other creditors?

Thanks for anyone who has information.