Rani, boy do some people have a nerve. Put your foot down on this one. To qualify for a one year extension the new employer would have had to request permission from the Immigration Dept. Given that they've sought this exemption they should bear the burden of paying for the air ticket both ways. They would have been advised at the time that the helper would have to go back within 12 months so that would have given them notice then that they would have to pay for a return ticket. Therefore, they accepted the burden of this in return for enjoying the immediate services of your helper.
In any event, it is a contractual provision between you and your helper that you're to provide a one way ticket, therefore only your helper can try to enforce this alleged right. She obviously wasn't going to ask for one after only just going back home. A person outside the contract, e.g. a new employer can't enforce this right. If they manage to make your previous helper seek the ticket fare, then you can counter claim for the cost of the ex gratia return airfare that you weren't legally required to give!
Even if they thought they had a leg to stand on and tried to enforce their supposed right to a one way fare if they approach Immigration they won't be remotely interested in helping them, especially if you show them some record - credit card statement, receipt from travel agent - showing that you'd given paid leave immediately before the end of the contract.
Good luck and don't be bullied!