Of course Louie would know Dewey would be a bad employee. If he knew about his brothers' first encounter with Mark Beaks, then he'd already know that he doesn't have the kind of dedication needed to be a good employee.
Scrooge butting in during Louie's interview with Roxanne to point out that Louie is an unpaid intern seems like money-grubbing semantics on Scrooge's part. However, Scrooge is likely well aware that having a minor in a major executive position would violate many, many child labor laws, so making that distinction is very important.
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