You will find that prostitutes are not the only ones falling victim to pervs and murderers.
It is a fact of life that scum like that exists, not only in HK but in every country in the world.
A couple of British girls were killed in Japan not so long ago, one found in a cave by the sea, covered in cement, the other in a flat in a bathtub full of sand.
They were not prostitutes as far as the news reported at the time.
Yes, a drop in centre for help and counseling would go a long way in that neck of the woods. Unfortunately the girls will not attend, not because of the "stigma", but the fears of missing tricks.
If prostitution was regulated like it is in Holland and some other parts of the world, with proper brothels and access to free medical help, and off the streets, the girls would be much safer.
But as long there are men, trawling for sex, there always be danger. Not just to working girls.
Just try not to worry too much, sounds like she knows what she is doing, and in my opinion she ain't too interested in a relationship of any sort with all that money rolling in...
Good luck.