You probably have a ton more expertise in these matters than I do so I'm sure you've thought of this but I'll throw it out there for consideration.
A) I'm sure there's a way he can contact his former employee. He may not want to but he can and he can at least ask what the pw was. I think, and I could be wrong, but intellectual property including passwords and so on have to be provided by the employee by law upon termination of employment either voluntarily or involuntarily.
B) He could contact Quickbooks and prove he's the owner of the business. Subsequently even if he doesn't have all of the information I think anything bought and installed on his computers (property) legally belongs to him. Skuzzy probably knows better than I.
(I bet I'm making a huge bellybutton out of myself unintentionally for a change)
I could do a little research tomorrow to help you out. If you need somebody to kill some time that has time to kill shoot me a pm and I'll do some poking around if you like.