Have you taken your maid for confirmatory test? The report usually includes screening test VDRL which false positive is up to 10-20% (which means the report shows positive yet the result is false alarm)
VDRL positive can be many different diseases other than syphilis, like rheumatoid athritis, SLE and etc,.
Feeing pity on her is one thing. Having sexual transmitted disease that proves that she is not capable in taking care of your kid is another thing. The reason in having to under go medical checkup is to make sure she is fit for work. Obviously she is not. You can fire her with no problem indicating the reason in your letter to immigration making sure she can't get another job in hk.
Imagine your kid getting that disease by accident, what can you do? Thus is not kidding and you are no social worker either
See if you can find a western doctor to write a letter. I have seen a termination letter with reference from a doctors letter, from a western employer indicating "unfit for work" when the helper has dermatitis. Compare to your case, you are much more serious than this. Western doctor are more out spoken than Chinese ones. I think.