I did check with Labor Dept for similar things. Labor Dept told me:
you send a letter (by register mail) to the address of the helper (the one specified in the contract). The letter specified that she did not return to home XXX (the date she suppose return to HK) and did not resume duty. You are now give her final notice, if she still no show up by xxx (1-2 days later then the date the letter suppose can arrive her home), then, you will treat it as that she resign from the position with effective from xxx (the date).
And then, send a copy of this letter as supporting to the Immigration Dept after the date. Besides, go to Labor Dept to claim for the short-notice.