coco,
that's horrible horrible!
I also paid Ms Wong(for the last "make-up monster helper"), for the first 3 months. And she would give me a receipt. So there was proof that I paid her salary.
Did you keep some kind of proof of payment?
Since you hired through the agency, you should be protected from this kind of claim. Just tell your helper you have proof, and it's with the agency (even if you don't have it.)
So she's already worked with you for 3 months?
I also was very stupid, I fired make-up monster on the 3rd month + 1 day, so I had to give her "15 days notice" money.