As Hawkwood says, it is the training agency that set the price for e-learning, and none of this money gets passed on to the chosen dive centre that students affiliate to for their water based training. For some agencies like PADI, this can be pricey ($130), whereas some agencies, like SSI offer it for free. You are paying for the convenience of learning at home and saving time spent in a dive centre classroom - as a dive centre based in the tropics we often get enquiries about taking e-learning so students can swap their classroom sessions for beach time! However, the costs for the dive centre are not dramatically reduced. E-learners still need to complete a review of the material with their instructor, and in most cases complete the final exam too, so they still need some land based instructor time as well as the bater based sections. Its the water portions of the course that are the most expensive - instructors time, pool space, equipment rental fees, boat captain, fuel costs etc etc - you get the picture. So there is not really a great deal the dive centre can do to reduce the price for you - other than a small deduction to make up for not having to purchase the full crew pack.
To be honest, if your gf has the time, I'd recommend sticking with the normal OW schedule including academics taught by her instructor. Its purely my own opinion as an instructor, but I'm not a huge fan of teaching online learning students. First of all you dont get the opportunity to build rapport and trust with your students before the waterwork sessions. Also the online learning, to the best of my knowledge, does not expose students to the full syllabus (ie a full online copy of the manual) that regular students get. As such, there are questions in the final exam that online learners do not know the answers to without further councelling from their instructor. Dont get me wrong, this can be true of the paper based manual knowledge reviews too, but thats the whole point of the instructor doing academic sessions - to fill in gaps, explain things more clearly, expand on important points and give examples and stories detailing the practical application of what you are learning into the open water environment. Basically ensuring that you have a thorough understanding of all the salient points. These are things which the online learner often misses out on.
Sorry, dont know any centres in Hong Konk, but good luck in your search and I hope your gf enjoys her course, whichever way she chooses to learn