It is the employer's obligation to send the helper back home when the contract expires.
Understand helpers may want $$ instead of ticket but cannot understand why employers want to pay $$ rather than buying the ticket. It is not to do the helper a favour but violates the contract terms.
If you want to do the helper a favour, you can buy the type of ticket that allows her to change the date with one year validity.
If she wants to work in another country, Hong Kong would not be her port of deployment. So, she may not be able to fly directly from Hong Kong to that country. Her new employer may not be allowed to pay for the ticket from Hong Kong to the country she is going to work in (suppose all employment is regulated).