If she has finished her existing contract (i.e she did not break her contract or she was not terminated [dreadful expression] surely she can stay in HK?
Even the girls who do get terminated or break their contracts normally stay in HK for a maximum of 2 weeks then exit to Macau (used to be China) and stay in a boarding house for a month or so whilst their new application is being processed.
If a DH finishes her contract there is, as I understand, it no need for her to exit HK as long as she either a) signs a new contract with the same employer or b) signs a new contract with a new employer and the application is being processed with the Immigration Dept before expiry of the old contract.
Many DHs process their new contracts 4-6 weeks before expiry of the old one.
A DH is, of course, perfectly at liberty to exit HK between contracts if she so wishes as the employer has to provide a return ticket as part of the contract (or at least provide the cash equivalent which is more the norm).