my wife bought a nail salon with $6 thousand in gift cards. can she get this back from previous owner?

Hard to answer your question without knowing more.  Did your wife know that the salon had sold gift cards before she bough the salon?  She'd be on notice, at least generally, that there's an "account payable" out there that she'd have to fulfill as the new owner.  if she actually didn't know about the gift cards, what did the written purchase agreement say, if anything, about disclosure of liabilities and responsiblity for them?