Actually, after she finishes her contract, her maximum stay in HK or the entry visa will expire by 14 days. She needs to extend it by herself by going to Immigration Department. Overstaying in HK is breaking the law. My suggestion is to get her an air-ticket going back on the 14th day after her last working day, get the type that is non-transferable and close ticket, so that she can't give it to her friend. In case you are afraid that you are breaking the law, countersign on 2 sets of receipt, ask your agent to countersign as witness as well, one set for you and one for her, make a photocopy of the air ticket and invoice, then there shouldn't be any problem.