You do not need to entertain her on the change of date. Employer's obligation is to provide return ticket upon completion of contract or premature termination as per the employment contract. The contract does not say the date should be at the discretion of the helper.
I think she cannot sue you on this. Please correct if I am wrong.
I am afraid if I buy her a specific date but later on she change her mind we will suffer the amendment fee, they have 14 days to stay in hk after termination. So is it better to buy a ticket with 14 days validity ? How to protect ourselves like to add a clause in the receipt that the maid have bear the amendment charges after 14 days ? Can they take the ticket to airline for refund ?