If your girlfriend is looking to teach English (as a second language) she should apply for a job once she gets to HK. They are easy to find and with her experience and background it should be no problem.
If she is looking to teach at an English school then she should apply from overseas.
Since your girlfriend has her PGCE and 5 years work experience she should definitely look into getting a job with the international schools. They pay very well and if she applies from overseas they may give her a better package than if she were to apply from HK.
Also, if she teaches at a school rather than at an English center she will get all the holidays the children get!
The ESF schools (
http://www.esf.edu.hk/) and all the international schools (French, German Swiss, Chinese, Hong Kong) - they all have English streams.
Hope this helps!