qualified theft in the workplace????

A colleague of my wife acting as teller in a pawnshop has had undermining the pawned items, replacing the original with decoy items to misled my wife who is acting in-charge that the items were intact, considering these are contained in a sealed envelopes. Only recently that she discovered such fraud when she prefers for the annual audit.

What should we do first to establish there is theft happening? What documents should we prefer? Is this a qualified theft case? Though, upon confrontation, the accused admitted her guilt and promised to pay the amount involved, however, it is already due.

Please advice.....