If your helper (i.e someone that you have contractually employed under a FDH Contract/Visa) is working for anyone else then she, you and her other employer are committing an offence. It seems from your post here that you connived in her offence.
It is absurdly stupid for an employer to file a tax return showing someone who is on a domestic helper visa as working for them! We all know that lots of helpers do other bits of work for cash on the side. For some the main contract actually pays nothing and their income is entirely other cash jobs. But those side jobs have to be cash-only and no paperwork!
If the employer has already filed the return to IRD then:
a) the employer is likely to be prosecuted
b) your FDH should probably pack and leave ASAP because if she is still here when this gets to the police (which it will) then she will be arrested and will do jail time (and then be deported anyway).
c) I hope you're a PR because you are likely to be prosecuted and, if not PR, any visa extensions refused.